Light-Emitting Element, Display Device, Electronic Device, and Lighting Device

ABSTRACT

To provide a light-emitting element with high emission efficiency and low driving voltage. The light-emitting element includes a guest material and a host material. A HOMO level of the guest material is higher than a HOMO level of the host material. An energy difference between the LUMO level and a HOMO level of the guest material is larger than an energy difference between the LUMO level and a HOMO level of the host material. The guest material has a function of converting triplet excitation energy into light emission. An energy difference between the LUMO level of the host material and the HOMO level of the guest material is larger than or equal to energy of light emission of the guest material.

This application is a continuation of copending U.S. application Ser.No. 15/277,323, filed on Sep. 27, 2016 which is incorporated herein byreference.

TECHNICAL FIELD

One embodiment of the present invention relates to a light-emittingelement, a display device including the light-emitting element, anelectronic device including the light-emitting element, and a lightingdevice including the light-emitting element.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, or a manufacturing method. In addition, one embodimentof the present invention relates to a process, a machine, manufacture,or a composition of matter. Specifically, examples of the technicalfield of one embodiment of the present invention disclosed in thisspecification include a semiconductor device, a display device, a liquidcrystal display device, a light-emitting device, a lighting device, apower storage device, a memory device, a method for driving any of them,and a method for manufacturing any of them.

BACKGROUND ART

In recent years, research and development have been extensivelyconducted on light-emitting elements using electroluminescence (EL). Ina basic structure of such a light-emitting element, a layer containing alight-emitting material (an EL layer) is interposed between a pair ofelectrodes. By applying a voltage between the pair of electrodes of thiselement, light emission from the light-emitting material can beobtained.

Since the above light-emitting element is of a self-luminous type, adisplay device using this light-emitting element has advantages such ashigh visibility, no necessity of a backlight, low power consumption, andthe like. Further, the display device also has advantages in that it canbe formed to be thin and lightweight, and has high response speed.

In a light-emitting element (e.g., an organic EL element) whose EL layercontains an organic material as a light-emitting material and isprovided between a pair of electrodes, application of a voltage betweenthe pair of electrodes causes injection of electrons from a cathode andholes from an anode into the EL layer having a light-emitting propertyand thus a current flows. By recombination of the injected electrons andholes, the organic material having a light-emitting property is broughtinto an excited state to provide light emission.

Note that an excited state formed by an organic material can be asinglet excited state (S*) or a triplet excited state (T*). Lightemission from the singlet excited state is referred to as fluorescence,and light emission from the triplet excited state is referred to asphosphorescence. The formation ratio of S* to T* in the light-emittingelement is 1:3. In other words, a light-emitting element including acompound emitting phosphorescence (phosphorescent compound) has higherlight emission efficiency than a light-emitting element including acompound emitting fluorescence (fluorescent compound). Therefore,light-emitting elements containing phosphorescent materials capable ofconverting energy of the triplet excited state into light emission havebeen actively developed in recent years (e.g., see Patent Document 1).

Energy for exciting an organic material depends on an energy differencebetween the LUMO level and the HOMO level of the organic material. Theenergy difference approximately corresponds to singlet excitationenergy. In a light-emitting element containing a phosphorescent organicmaterial, triplet excitation energy is converted into light emissionenergy. Thus, when the energy difference between the singlet excitedstate and the triplet excited state of an organic material is large, theenergy needed for exciting the organic material is higher than the lightemission energy by the amount corresponding to the energy difference.The difference between the energy for exciting the organic material andthe light emission energy affects element characteristics of alight-emitting element: the driving voltage of the light-emittingelement increases. Research and development are being conducted ontechniques for reducing the driving voltage (see Patent Document 2).

Among light-emitting elements including phosphorescent materials, alight-emitting element that emits blue light in particular has not yetbeen put into practical use because it is difficult to develop a stableorganic material having a high triplet excited energy level. This hasmotivated the research effort to develop highly reliable light-emittingelements that exhibit phosphorescence with high emission efficiency.

REFERENCES Patent Documents [Patent Document 1] Japanese PublishedPatent Application No. 2010-182699 [Patent Document 2] JapanesePublished Patent Application No. 2012-212879 DISCLOSURE OF INVENTION

An iridium complex is known as a phosphorescent material with highemission efficiency. An iridium complex including a pyridine skeleton ora nitrogen-containing five-membered heterocyclic skeleton as a ligand isknown as an iridium complex with high light emission energy. Althoughthe pyridine skeleton and the nitrogen-containing five-memberedheterocyclic skeleton have high triplet excitation energy, they havepoor electron-accepting property. Accordingly, the HOMO level and LUMOlevel of the iridium complex having these skeletons as a ligand arehigh, and hole carriers are easily injected thereto, while electroncarriers are not. Thus, in the iridium complex with high light emissionenergy, excitation of carriers by direct carrier recombination isdifficult, which means that the efficient light emission is difficult.

In view of the above, an object of one embodiment of the presentinvention is to provide a light-emitting element that has high emissionefficiency and contains a phosphorescent material. Another object of oneembodiment of the present invention is to provide a light-emittingelement with low power consumption. Another object of one embodiment ofthe present invention is to provide a light-emitting element with highreliability. Another object of one embodiment of the present inventionis to provide a novel light-emitting element. Another object of oneembodiment of the present invention is to provide a novel light-emittingdevice. Another object of one embodiment of the present invention is toprovide a novel display device.

Note that the description of the above object does not disturb theexistence of other objects. In one embodiment of the present invention,there is no need to achieve all the objects. Other objects are apparentfrom and can be derived from the description of the specification andthe like.

One embodiment of the present invention is a light-emitting elementincluding a host material that can efficiently excite a phosphorescentmaterial.

One embodiment of the present invention is a light-emitting elementwhich includes a guest material and a host material and in which a HOMOlevel of the guest material is higher than a HOMO level of the hostmaterial, an energy difference between a LUMO level of the guestmaterial and the HOMO level of the guest material is larger than anenergy difference between a LUMO level of the host material and the HOMOlevel of the host material, and the guest material has a function ofconverting triplet excitation energy into light emission.

One embodiment of the present invention is a light-emitting elementwhich includes a guest material and a host material and in which a HOMOlevel of the guest material is higher than a HOMO level of the hostmaterial, an energy difference between a LUMO level of the guestmaterial and the HOMO level of the guest material is larger than anenergy difference between a LUMO level of the host material and the HOMOlevel of the host material, the guest material has a function ofconverting triplet excitation energy into light emission, and an energydifference between the LUMO level of the host material and the HOMOlevel of the guest material is larger than or equal to transition energycalculated from an absorption edge of an absorption spectrum of theguest material.

One embodiment of the present invention is a light-emitting elementwhich includes a guest material and a host material and in which a HOMOlevel of the guest material is higher than a HOMO level of the hostmaterial, an energy difference between a LUMO level of the guestmaterial and the HOMO level of the guest material is larger than anenergy difference between a LUMO level of the host material and the HOMOlevel of the host material, the guest material has a function ofconverting triplet excitation energy into light emission, and an energydifference between the LUMO level of the host material and the HOMOlevel of the guest material is larger than or equal to light emissionenergy of the guest material.

In each of the above structures, it is preferable that the energydifference between the LUMO level of the guest material and the HOMOlevel of the guest material be larger than the transition energycalculated from the absorption edge of the absorption spectrum of theguest material by 0.4 eV or more. It is preferable that the energydifference between the LUMO level of the guest material and the HOMOlevel of the guest material be larger than the light emission energy ofthe guest material by 0.4 eV or more.

In each of the above structures, it is preferable that the host materialhave a difference between a singlet excitation energy level and atriplet excitation energy level of larger than 0 eV and smaller than orequal to 0.2 eV. It is preferable that the host material have a functionof exhibiting thermally activated delayed fluorescence.

In each of the above structures, it is preferable that the host materialhave a function of supplying excitation energy to the guest material. Itis preferable that an emission spectrum of the host material include awavelength region overlapping with an absorption band on the lowestenergy side in the absorption spectrum of the guest material.

In each of the above structures, it is preferable that the guestmaterial include iridium. It is preferable that the guest material emitlight.

In each of the above structures, it is preferable that the host materialhave a function of transporting an electron. It is preferable that thehost material have a function of transporting a hole. It is preferablethat the host material include a π-electron deficient heteroaromaticring skeleton and include at least one of a π-electron richheteroaromatic ring skeleton and an aromatic amine skeleton. It ispreferable that the π-electron deficient heteroaromatic ring skeletoninclude at least one of a diazine skeleton and a triazine skeleton andthe π-electron rich heteroaromatic ring skeleton include at least one ofan acridine skeleton, a phenoxazine skeleton, a phenothiazine skeleton,a furan skeleton, a thiophene skeleton, and a pyrrole skeleton.

One embodiment of the present invention is a display device includingthe light-emitting element having any of the above structures, and atleast one of a color filter and a transistor. One embodiment of thepresent invention is an electronic device including the above-describeddisplay device and at least one of a housing and a touch sensor. Oneembodiment of the present invention is a lighting device including thelight-emitting element having any of the above structures, and at leastone of a housing and a touch sensor. The category of one embodiment ofthe present invention includes not only a light-emitting deviceincluding a light-emitting element but also an electronic deviceincluding a light-emitting device. Therefore, the light-emitting devicein this specification refers to an image display device or a lightsource (e.g., a lighting device). A display module in which a connectorsuch as a flexible printed circuit (FPC) or a tape carrier package (TCP)is connected to a light-emitting device, a display module in which aprinted wiring board is provided on the tip of a TCP, and a displaymodule in which an integrated circuit (IC) is directly mounted on alight-emitting element by a chip on glass (COG) method are alsoembodiments of the present invention.

With one embodiment of the present invention, a light-emitting elementthat has high emission efficiency and contains a phosphorescent materialis provided. With one embodiment of the present invention, alight-emitting element with low power consumption is provided. With oneembodiment of the present invention, a light-emitting element with highreliability is provided. With one embodiment of the present invention, anovel light-emitting element is provided. With one embodiment of thepresent invention, a novel light-emitting device is provided. With oneembodiment of the present invention, a novel display device can beprovided.

Note that the description of the above effects does not disturb theexistence of other effects. In one embodiment of the present invention,there is no need to achieve all the effects. Other effects are apparentfrom and can be derived from the description of the specification, thedrawings, the claims, and the like.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are schematic cross-sectional views of a light-emittingelement of one embodiment of the present invention.

FIGS. 2A and 2B are schematic views showing a correlation of energylevels and a correlation between energy bands in a light-emitting layerof a light-emitting element of one embodiment of the present invention.

FIGS. 3A and 3B are schematic cross-sectional views of a light-emittingelement of one embodiment of the present invention.

FIGS. 4A and 4B are schematic views showing a correlation between energylevels and a correlation between energy bands in a light-emitting layerof a light-emitting element of one embodiment of the present invention.

FIGS. 5A and 5B are schematic cross-sectional views of a light-emittingelement of one embodiment of the present invention and FIG. 5C is aschematic view showing a correlation between energy levels in alight-emitting layer.

FIGS. 6A and 6B are schematic cross-sectional views of a light-emittingelement of one embodiment of the present invention and FIG. 6C is aschematic view showing a correlation between energy levels in alight-emitting layer.

FIGS. 7A and 7B are each a schematic cross-sectional view of alight-emitting element of one embodiment of the present invention.

FIGS. 8A and 8B are each a schematic cross-sectional view of alight-emitting element of one embodiment of the present invention.

FIGS. 9A to 9C are schematic cross-sectional views illustrating a methodfor manufacturing a light-emitting element of one embodiment of thepresent invention.

FIGS. 10A to 10C are schematic cross-sectional views illustrating themethod for manufacturing a light-emitting element of one embodiment ofthe present invention.

FIGS. 11A and 11B are a top view and a schematic cross-sectional viewillustrating a display device of one embodiment of the presentinvention.

FIGS. 12A and 12B are each a schematic cross-sectional view illustratinga display device of one embodiment of the present invention.

FIG. 13 is a schematic cross-sectional view illustrating a displaydevice of one embodiment of the present invention.

FIGS. 14A and 14B are schematic cross-sectional views each illustratinga display device of one embodiment of the present invention.

FIGS. 15A and 15B are schematic cross-sectional views illustrating adisplay device of one embodiment of the present invention.

FIG. 16 is a schematic cross-sectional view illustrating a displaydevice of one embodiment of the present invention.

FIGS. 17A and 17B are each a schematic cross-sectional view illustratinga display device of one embodiment of the present invention.

FIG. 18 is a schematic cross-sectional view illustrating a displaydevice of one embodiment of the present invention.

FIGS. 19A and 19B are each a schematic cross-sectional view illustratinga display device of one embodiment of the present invention.

FIGS. 20A and 20B are a block diagram and a circuit diagram illustratinga display device of one embodiment of the present invention.

FIGS. 21A and 21B are circuit diagrams each illustrating a pixel circuitof a display device of one embodiment of the present invention.

FIGS. 22A and 22B are circuit diagrams each illustrating a pixel circuitof a display device of one embodiment of the present invention.

FIGS. 23A and 23B are perspective views of an example of a touch panelof one embodiment of the present invention.

FIGS. 24A to 24C are cross-sectional views of examples of a displaydevice and a touch sensor of one embodiment of the present invention.

FIGS. 25A and 25B are cross-sectional views each illustrating an exampleof a touch panel of one embodiment of the present invention.

FIGS. 26A and 26B are a block diagram and a timing chart of a touchsensor of one embodiment of the present invention.

FIG. 27 is a circuit diagram of a touch sensor of one embodiment of thepresent invention.

FIG. 28 is a perspective view illustrating a display module of oneembodiment of the present invention.

FIGS. 29A to 29G illustrate electronic devices of one embodiment of thepresent invention.

FIGS. 30A to 30F illustrate electronic devices of one embodiment of thepresent invention.

FIGS. 31A to 31D illustrate electronic devices of one embodiment of thepresent invention.

FIGS. 32A and 32B are perspective views illustrating a display device ofone embodiment of the present invention.

FIGS. 33A to 33C are a perspective view and cross-sectional viewsillustrating light-emitting devices of one embodiment of the presentinvention.

FIGS. 34A to 34D are each a cross-sectional view illustrating alight-emitting device of one embodiment of the present invention.

FIGS. 35A to 35C illustrate an electronic device and a lighting deviceof one embodiment of the present invention.

FIG. 36 illustrates lighting devices of one embodiment of the presentinvention.

FIG. 37 is a schematic cross-sectional view illustrating alight-emitting element in Example.

FIG. 38 shows the current efficiency vs. luminance characteristics oflight-emitting elements in Example.

FIG. 39 shows luminance vs. voltage characteristics of light-emittingelements in Example.

FIG. 40 shows the external quantum efficiency vs. luminancecharacteristics of light-emitting elements in Example.

FIG. 41 shows power efficiency vs. luminance characteristics oflight-emitting elements in Example.

FIG. 42 shows electroluminescence spectra of light-emitting elements inExample.

FIG. 43 shows emission spectra of a host material in Example.

FIG. 44 shows transient fluorescence characteristics of a host materialin Example.

FIG. 45 shows an absorption spectrum and an emission spectrum of a guestmaterial in Example.

FIG. 46 shows current efficiency vs. luminance characteristics oflight-emitting elements in Example.

FIG. 47 shows luminance vs. voltage characteristics of light-emittingelements in Example.

FIG. 48 shows external quantum efficiency vs. luminance characteristicsof light-emitting elements in Example.

FIG. 49 shows power efficiency vs. luminance characteristics oflight-emitting elements in Example.

FIG. 50 shows electroluminescence spectra of light-emitting elements inExample.

FIG. 51 shows current efficiency vs. luminance characteristics of alight-emitting element in Example.

FIG. 52 shows luminance vs. voltage characteristics of a light-emittingelement in Example.

FIG. 53 shows external quantum efficiency vs. luminance characteristicsof a light-emitting element in Example.

FIG. 54 shows power efficiency vs. luminance characteristics of alight-emitting element in Example.

FIG. 55 shows an electroluminescence spectrum of a light-emittingelement in Example.

FIG. 56 shows an absorption spectrum and an emission spectrum of a guestmaterial in Example.

FIG. 57 shows current efficiency vs. luminance characteristics of alight-emitting element in Example.

FIG. 58 shows luminance vs. voltage characteristics of a light-emittingelement in Example.

FIG. 59 shows external quantum efficiency vs. luminance characteristicsof a light-emitting element in Example.

FIG. 60 shows power efficiency vs. luminance characteristics of alight-emitting element in Example.

FIG. 61 shows an electroluminescence spectrum of a light-emittingelement in Example.

FIG. 62 shows emission spectra of a host material in Example.

FIGS. 63A and 63B show transient fluorescence characteristics of a hostmaterial in Example.

FIG. 64 shows current efficiency vs. luminance characteristics of alight-emitting element in Example.

FIG. 65 shows luminance vs. voltage characteristics of a light-emittingelement in Example.

FIG. 66 shows external quantum efficiency vs. luminance characteristicsof a light-emitting element in Example.

FIG. 67 shows power efficiency vs. luminance characteristics of alight-emitting element in Example.

FIG. 68 shows an electroluminescence spectrum of a light-emittingelement in Example.

FIG. 69 shows current efficiency vs. luminance characteristics of alight-emitting element in Example.

FIG. 70 shows the luminance vs. voltage characteristics of alight-emitting element in Example.

FIG. 71 shows the external quantum efficiency vs. luminancecharacteristics of a light-emitting element in Example.

FIG. 72 shows power efficiency vs. luminance characteristics of alight-emitting element in Example.

FIG. 73 shows an electroluminescence spectrum of a light-emittingelement in Example.

FIG. 74 shows emission spectra of a host material in Example.

FIG. 75 shows an absorption spectrum and an emission spectrum of a guestmaterial in Example.

FIG. 76 shows current efficiency vs. luminance characteristics of alight-emitting element in Example.

FIG. 77 shows luminance vs. voltage characteristics of a light-emittingelement in Example.

FIG. 78 shows external quantum efficiency vs. luminance characteristicsof a light-emitting element in Example.

FIG. 79 shows power efficiency vs. luminance characteristics of alight-emitting element in Example.

FIG. 80 shows an electroluminescence spectrum of a light-emittingelement in Example.

FIG. 81 shows emission spectra of a host material in Example.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described in detail belowwith reference to the drawings. However, the present invention is notlimited to description to be given below, and modes and details thereofcan be variously modified without departing from the purpose and thescope of the present invention. Accordingly, the present inventionshould not be interpreted as being limited to the content of theembodiments below.

Note that the position, the size, the range, or the like of eachstructure illustrated in drawings and the like is not accuratelyrepresented in some cases for simplification. Therefore, the disclosedinvention is not necessarily limited to the position, the size, therange, or the like disclosed in the drawings and the like.

Note that the ordinal numbers such as “first”, “second”, and the like inthis specification and the like are used for convenience and do notdenote the order of steps or the stacking order of layers. Therefore,for example, description can be made even when “first” is replaced with“second” or “third”, as appropriate. In addition, the ordinal numbers inthis specification and the like are not necessarily the same as thosewhich specify one embodiment of the present invention.

In the description of modes of the present invention in thisspecification and the like with reference to the drawings, the samecomponents in different diagrams are commonly denoted by the samereference numeral in some cases.

In this specification and the like, the terms “film” and “layer” can beinterchanged with each other. For example, the term “conductive layer”can be changed into the term “conductive film” in some cases. Also, theterm “insulating film” can be changed into the term “insulating layer”in some cases.

In this specification and the like, a singlet excited state (S*) refersto a singlet state having excitation energy. An S1 level means thelowest level of the singlet excitation energy level, that is, theexcitation energy level of the lowest singlet excited state. A tripletexcited state (T*) refers to a triplet state having excitation energy. AT1 level means the lowest level of the triplet excitation energy level,that is, the excitation energy level of the lowest triplet excitedstate. Note that in this specification and the like, a singlet excitedstate and a singlet excitation energy level mean the lowest singletexcited state and the S1 level, respectively, in some cases. A tripletexcited state and a triplet excitation energy level mean the lowesttriplet excited state and the T1 level, respectively, in some cases.

In this specification and the like, a fluorescent material refers to amaterial that emits light in the visible light region when therelaxation from the singlet excited state to the ground state occurs. Aphosphorescent material refers to a material that emits light in thevisible light region at room temperature when the relaxation from thetriplet excited state to the ground state occurs. That is, aphosphorescent material refers to a material that can convert tripletexcitation energy into visible light.

Phosphorescence emission energy or a triplet excitation energy can beobtained from a wavelength of an emission peak (including a shoulder) ora rising portion on the shortest wavelength side of phosphorescenceemission. Note that the phosphorescence emission can be observed bytime-resolved photoluminescence in a low-temperature (e.g., 10 K)environment. A thermally activated delayed fluorescence emission energycan be obtained from a wavelength of an emission peak (including ashoulder) or a rising portion on the shortest wavelength side ofthermally activated delayed fluorescence.

Note that in this specification and the like, “room temperature” refersto a temperature higher than or equal to 0° C. and lower than or equalto 40° C.

In this specification and the like, a wavelength range of blue refers toa wavelength range of greater than or equal to 400 nm and less than 500nm, and blue light has at least one peak in that range in an emissionspectrum. A wavelength range of green refers to a wavelength range ofgreater than or equal to 500 nm and less than 580 nm, and green lighthas at least one peak in that range in an emission spectrum. Awavelength range of red refers to a wavelength range of greater than orequal to 580 nm and less than or equal to 680 nm, and red light has atleast one peak in that range in an emission spectrum.

Embodiment 1

In this embodiment, a light-emitting element of one embodiment of thepresent invention will be described below with reference to FIGS. 1A and1B, FIGS. 2A and 2B, FIGS. 3A and 3B, and FIGS. 4A and 4B.

Structure Example 1 of Light-Emitting Element

First, a structure of the light-emitting element of one embodiment ofthe present invention will be described below with reference to FIGS. 1Aand 1B.

FIG. 1A is a schematic cross-sectional view of a light-emitting element150 of one embodiment of the present invention.

The light-emitting element 150 includes a pair of electrodes (anelectrode 101 and an electrode 102) and an EL layer 100 between the pairof electrodes. The EL layer 100 includes at least a light-emitting layer130.

The EL layer 100 illustrated in FIG. 1A includes functional layers suchas a hole-injection layer 111, a hole-transport layer 112, anelectron-transport layer 118, and an electron-injection layer 119 inaddition to the light-emitting layer 130.

In this embodiment, although description is given assuming that theelectrode 101 and the electrode 102 of the pair of electrodes serve asan anode and a cathode, respectively, they are not limited thereto forthe structure of the light-emitting element 150. That is, the electrode101 may be a cathode, the electrode 102 may be an anode, and thestacking order of the layers between the electrodes may be reversed. Inother words, the hole-injection layer 111, the hole-transport layer 112,the light-emitting layer 130, the electron-transport layer 118, and theelectron-injection layer 119 may be stacked in this order from the anodeside.

The structure of the EL layer 100 is not limited to the structureillustrated in FIG. 1A, and a structure including at least one layerselected from the hole-injection layer 111, the hole-transport layer112, the electron-transport layer 118, and the electron-injection layer119 may be employed. Alternatively, the EL layer 100 may include afunctional layer which is capable of lowering a hole- orelectron-injection barrier, improving a hole- or electron-transportproperty, diminishing a hole- or electron-transport property, orsuppressing a quenching phenomenon by an electrode, for example. Notethat the functional layers may each be a single layer or stacked layers.

FIG. 1B is a schematic cross-sectional view illustrating an example ofthe light-emitting layer 130 in FIG. 1A. The light-emitting layer 130 inFIG. 1B includes a guest material 131 and a host material 132.

In the light-emitting layer 130, the host material 132 is present in thelargest proportion by weight, and the guest material 131 is dispersed inthe host material 132.

The guest material 131 is a light-emitting organic material. Thelight-emitting organic material preferably has a function of convertingtriplet excitation energy into light emission and is preferably amaterial capable of exhibiting phosphorescence (hereinafter alsoreferred to as a phosphorescent material). In the description below, aphosphorescent material is used as the guest material 131. The guestmaterial 131 may be rephrased as the phosphorescent material.

<Light Emission Mechanism 1 of Light-Emitting Element>

Next, the light emission mechanism of the light-emitting layer 130 isdescribed below.

In the light-emitting element 150 of one embodiment of the presentinvention, voltage application between the pair of electrodes (theelectrodes 101 and 102) causes electrons and holes to be injected fromthe cathode and the anode, respectively, into the EL layer 100 and thuscurrent flows. By recombination of the injected electrons and holes, theguest material 131 in the light-emitting layer 130 of the EL layer 100is brought into an excited state to provide light emission.

Note that light emission from the guest material 131 can be obtainedthrough the following two processes:

(α) direct recombination process; and

(β) energy transfer process.

<<(α) Direct Recombination Process>>

First, the direct recombination process in the guest material 131 willbe described. Carriers (electrons and holes) are recombined in the guestmaterial 131, and the guest material 131 is brought into an excitedstate. In this case, energy for exciting the guest material 131 by thedirect carrier recombination process depends on the energy differencebetween the lowest unoccupied molecular orbital (LUMO) level and thehighest occupied molecular orbital (HOMO) level of the guest material131, and the energy difference approximately corresponds to singletexcitation energy. Since the guest material 131 is a phosphorescentmaterial, triplet excitation energy is converted into light emission.Thus, when the energy difference between the singlet excited state andthe triplet excited state of the guest material 131 is large, the energyfor exciting the guest material 131 is higher than the light emissionenergy by the amount corresponding to the energy difference.

The energy difference between the energy for exciting the guest material131 and the light emission energy affects element characteristics of alight-emitting element: the driving voltage of the light-emittingelement varies. Thus, in (a) direct recombination process, the lightemission start voltage of the light-emitting element is higher than thevoltage corresponding to the light emission energy in the guest material131.

In the case where the guest material 131 has high light emission energy,the guest material 131 has a high LUMO level. Thus, the injection ofelectrons as carriers into the guest material 131 is hampered, and thedirect recombination of carriers (electrons and holes) is less likely tooccur in the guest material 131. Accordingly, high emission efficiencyis hardly obtained in the light-emitting element.

<<(β) Energy Transfer Process>>

Next, in order to describe the energy transfer process of the hostmaterial 132 and the guest material 131, a schematic diagramillustrating the correlation of energy levels is shown in FIG. 2A. Thefollowing explains what terms and signs in FIG. 2A represent:

Guest (131): the guest material 131 (the phosphorescent material);

Host (132): the host material 132;

S_(G): an S1 level of the guest material 131 (the phosphorescentmaterial);

T_(G): a T1 level of the guest material 131 (the phosphorescentmaterial);

S_(H): an S1 level of the host material 132; and

T_(H): a T1 level of the host material 132.

In the case where carriers are recombined in the host material 132 andthe singlet excited state and the triplet excited state of the hostmaterial 132 are formed, as shown in Route E₁ and Route E₂ in FIG. 2A,both of the singlet excitation energy and the triplet excitation energyof the host material 132 are transferred from the singlet excitationenergy level (S_(H)) and the triplet excitation energy level (T_(H)) ofthe host material 132 to the triplet excitation energy level (T_(G)) ofthe guest material 131, and the guest material 131 is brought into atriplet excited state. Phosphorescence is obtained from the guestmaterial 131 in the triplet excited state.

Note that both of the singlet excitation energy level (S_(H)) and thetriplet excitation energy level (T_(H)) of the host material 132 arepreferably higher than or equal to the triplet excitation energy level(T_(G)) of the guest material 131. In that case, the singlet excitationenergy and the triplet excitation energy generated in the host material132 can be efficiently transferred from the singlet excitation energylevel (S_(H)) and the triplet excitation energy level (T_(H)) of thehost material 132 to the triplet excitation energy level (T_(G)) of theguest material 131.

In other words, in the light-emitting layer 130, excitation energy istransferred from the host material 132 to the guest material 131.

Note that in the case where the light-emitting layer 130 includes thehost material 132, the guest material 131, and a material other than thehost material 132 and the guest material 131, the material other thanthe host material 132 and the guest material 131 in the light-emittinglayer 130 preferably has a triplet excitation energy level higher thanthe triplet excitation energy level (T_(H)) of the host material 132.Thus, quenching of the triplet excitation energy of the host material132 is less likely to occur, which causes efficient energy transfer tothe guest material 131.

In order to reduce energy loss caused when the singlet excitation energyof the host material 132 is transferred to the triplet excitation energylevel (T_(G)) of the guest material 131, it is preferable that theenergy difference between the singlet excitation energy level (S_(H))and the triplet excitation energy level (T_(H)) of the host material 132be small.

FIG. 2B is an energy band diagram of the guest material 131 and the hostmaterial 132. In FIG. 2B, “Guest (131)” represents the guest material131, “Host (132)” represents the host material 132, ΔE_(G) representsthe energy difference between the LUMO level and the HOMO level of theguest material 131, ΔE_(H) represents the energy difference between theLUMO level and the HOMO level of the host material 132, and ΔE_(B)represents the energy difference between the LUMO level of the hostmaterial 132 and the HOMO level of the guest material 131.

To make the guest material 131 emit light of a short wavelength and withhigh emission energy, the larger the energy difference (ΔE_(G)) betweenthe LUMO level and the HOMO level of the guest material 131 is, thebetter. However, excitation energy in the light-emitting element 150 ispreferably as small as possible in order to reduce the driving voltage;thus, the smaller the excitation energy of an excited state formed bythe host material 132 is, the better. Therefore, the energy difference(ΔE_(H)) between the LUMO level and the HOMO level of the host material132 is preferably small.

The guest material 131 is a phosphorescent material and thus has afunction of converting triplet excitation energy into light emission. Inaddition, energy is more stable in a triplet excited state than in asinglet excited state. Thus, the guest material 131 can emit lighthaving energy smaller than the energy difference (ΔE_(G)) between theLUMO level and the HOMO level of the guest material 131. The presentinventors have found out that even in the case where the energydifference (ΔE_(G)) between the LUMO level and the HOMO level of theguest material 131 is larger than the energy difference (ΔE_(H)) betweenthe LUMO level and the HOMO level of the host material 132, excitationenergy transfer from an excited state of the host material 132 to theguest material 131 is possible and light emission can be obtained fromthe guest material 131 as long as light emission energy (abbreviation:ΔE_(Em)) of the guest material 131 or transition energy (abbreviation:ΔE_(abs)) calculated from an absorption edge of an absorption spectrumof the guest material 131 is equivalent to or lower than ΔE_(H). WhenΔE_(G) of the guest material 131 is larger than the light emissionenergy (ΔE_(Em)) of the guest material 131 or the transition energy(ΔE_(abs)) calculated from the absorption edge of the absorptionspectrum of the guest material 131, high electrical energy thatcorresponds to ΔE_(G) is necessary to directly cause electricalexcitation of the guest material 131 and thus the driving voltage of thelight-emitting element is increased. However, in one embodiment of thepresent invention, the host material 132 is electrically excited withelectrical energy that corresponds to ΔE_(H) (that is smaller thanΔE_(G)), and the guest material 131 is brought into an excited state byenergy transfer therefrom, so that light emission of the guest material131 can be obtained with low driving voltage and high efficiency.Therefore, the light emission start voltage (a voltage at the time whenthe luminance exceeds 1 cd/m²) of the light-emitting element of oneembodiment of the present invention can be lower than the voltagecorresponding to the light emission energy (ΔE_(Em)) of the guestmaterial. That is, one embodiment of the present invention is usefulparticularly in the case where ΔE_(G) is significantly larger than thelight emission energy (ΔE_(Em)) of the guest material 131 or thetransition energy (ΔE_(abs)) calculated from the absorption edge of theabsorption spectrum of the guest material 131 (for example, in the casewhere the guest material is a blue light-emitting material). Note thatthe light emission energy (ΔE_(Em)) can be derived from a wavelength ofan emission peak (the maximum value, or including a shoulder) on theshortest wavelength side or a wavelength of a rising portion of theemission spectrum.

Note that in the case where the guest material 131 includes a heavymetal, intersystem crossing between a singlet state and a triplet stateis promoted by spin-orbit interaction (interaction between spin angularmomentum and orbital angular momentum of an electron), and transitionbetween a singlet ground state and a triplet excited state of the guestmaterial 131 is allowed in some cases. Therefore, the emissionefficiency and the absorption probability which relate to the transitionbetween the singlet ground state and the triplet excited state of theguest material 131 can be increased. Accordingly, the guest material 131preferably includes a metal element with large spin-orbit interaction,specifically a platinum group element (ruthenium (Ru), rhodium (Rh),palladium (Pd), osmium (Os), iridium (Ir), or platinum (Pt)). Inparticular, iridium is preferred because the absorption probability thatrelates to direct transition between a singlet ground state and atriplet excited state can be increased.

In order that the guest material 131 can emit light with a high lightemission energy (light of a short wavelength), the lowest tripletexcitation energy level of the guest material 131 is preferably high. Tomake the lowest triplet excitation energy level of the guest material131 high, a ligand coordinated to a heavy metal atom of the guestmaterial 131 preferably has a high lowest triplet excitation energylevel, a low electron-accepting property, and a high LUMO level.

Such a guest material tends to have a molecular structure having a highHOMO level and a high hole-accepting property. When the guest material131 has a molecular structure having a high hole-accepting property, theHOMO level of the guest material 131 is sometimes higher than that ofthe host material 132. In addition, when ΔE_(G) is larger than ΔE_(H),the LUMO level of the guest material 131 is higher than the LUMO levelof the host material 132. Note that the energy difference between theLUMO level of the guest material 131 and the LUMO level of the hostmaterial 132 is larger than the energy difference between the HOMO levelof the guest material 131 and the HOMO level of the host material 132.

Here, when the HOMO level of the guest material 131 is higher than thatof the host material 132 and the LUMO level of the guest material 131 ishigher than that of the host material 132, among carriers (holes andelectrons) injected from the pair of electrodes (the electrode 101 andthe electrode 102), holes injected from the anode are easily injected tothe guest material 131 and electrons injected from the cathode areeasily injected to the host material 132 in the light-emitting layer130. Therefore, the guest material 131 and the host material 132 form anexciplex in some cases. Particularly when the energy difference (ΔE_(B))between the LUMO level of the host material 132 and the HOMO level ofthe guest material 131 becomes smaller than the emission energy of theguest material 131 (ΔE_(Em)), generation of exciplexes formed by theguest material 131 and the host material 132 becomes predominant. Insuch a case, the guest material 131 itself is less likely to form anexcited state, which decreases emission efficiency of the light-emittingelement.

Note that the reactions described above can be expressed by GeneralFormula (G11) or (G12).

H⁻+G⁺→(H.G)*  (G11)

H+G*→(H.G)*  (G12)

General Formula (G11) represents a reaction in which the host material132 accepts an electron (H⁻) and the guest material 131 accepts a hole(G⁺), whereby the host material 132 and the guest material 131 form anexciplex ((H.G)*). General Formula (G12) represents a reaction in whichthe guest material 131 (G*) in the excited state interacts with the hostmaterial 132 (H) in the ground state, whereby the host material 132 andthe guest material 131 form an exciplex ((H.G)*). Formation of theexciplex ((H.G)*) by the host material 132 and the guest material 131makes it difficult to form an excited state (G*) of the guest material131 alone.

An exciplex formed by the host material 132 and the guest material 131has excitation energy that approximately corresponds to the energydifference (ΔE_(B)) between the LUMO level of the host material 132 andthe HOMO level of the guest material 131. The present inventors havefound that when the energy difference (ΔE_(B)) between the LUMO level ofthe host material 132 and the HOMO level of the guest material 131 islarger than or equal to an emission energy (ΔE_(Em)) of the guestmaterial 131 or a transition energy (ΔE_(abs)) calculated from theabsorption edge of the absorption spectrum of the guest material 131,the reaction for forming an exciplex by the host material 132 and theguest material 131 can be inhibited and thus light emission from theguest material 131 can be obtained efficiently. At this time, becauseΔE_(abs) is smaller than ΔE_(B), the guest material 131 easily receivesan excitation energy. Excitation of the guest material 131 by receptionof the excitation energy needs lower energy and provides a more stableexcitation state than formation of an exciplex by the host material 132and the guest material 131.

As described above, even when the energy difference (ΔE_(G)) between theLUMO level and the HOMO level of the guest material 131 is larger thanthe energy difference (ΔE_(H)) between the LUMO level and the HOMO levelof the host material 132, excitation energy transfers efficiently fromthe host material 132 in an excited state to the guest material 131 aslong as transition energy (ΔE_(abs)) calculated from the absorption edgeof the absorption spectrum of the guest material 131 is equivalent to orsmaller than ΔE_(H). As a result, a light-emitting element with highemission efficiency and low driving voltage can be obtained, which is afeature of one embodiment of the present invention. In this case, theformula ΔE_(G)>ΔE_(H)≥ΔE_(abs) (ΔE_(G) is larger than ΔE_(H) and ΔE_(H)is larger than or equal to ΔE_(abs)) is satisfied. Therefore, themechanism of one embodiment of the present invention is suitable in thecase where the energy difference (ΔE_(G)) between the LUMO level and theHOMO level of the guest material 131 is larger than the transitionenergy (ΔE_(abs)) calculated from the absorption edge of the absorptionspectrum of the guest material 131. Specifically, the energy difference(ΔE_(G)) between the LUMO level and the HOMO level of the guest material131 is preferably larger than the transition energy (ΔE_(abs))calculated from the absorption edge of the absorption spectrum of theguest material 131 by 0.3 eV or more, more preferably larger than thatby 0.4 eV or more. Since the light emission energy (ΔE_(Em)) of theguest material 131 is equivalent to or smaller than ΔE_(abs), the energydifference (ΔE_(G)) between the LUMO level and the HOMO level of theguest material 131 is preferably larger than the light emission energy(ΔE_(Em)) of the guest material 131 by 0.3 eV or more, more preferablylarger than that by 0.4 eV or more.

Furthermore, when the HOMO level of the guest material 131 is higherthan the HOMO level of the host material 132, it is preferable that theformula ΔE_(B)≥ΔE_(abs) (ΔE_(B) is larger than or equal to ΔE_(abs)) orΔE_(B)≥ΔE_(Em) (ΔE_(B) is larger than or equal to ΔE_(Em)) be satisfied.Therefore, it is preferable that the formulaΔE_(G)>ΔE_(H)>ΔE_(B)≥ΔE_(abs) (ΔE_(G) is larger than ΔE_(H), ΔE_(H) islarger than ΔE_(B), and ΔE_(B) is larger than or equal to ΔE_(abs)) orthe formula ΔE_(G)>ΔE_(H)>ΔE_(B)≥ΔE_(Em) (ΔE_(G) is larger than ΔE_(H),ΔE_(H) is larger than ΔE_(B), and ΔE_(B) is larger than or equal toΔE_(Em)) be satisfied. The above conditions are also importantdiscoveries in one embodiment of the present invention.

The energy difference (ΔE_(H)) between the LUMO level and the HOMO levelof the host material 132 is equivalent to or slightly larger than thesinglet excitation energy level (S_(H)) of the host material 132. Thesinglet excitation energy level (S_(H)) of the host material 132 ishigher than the triplet excitation energy level (T_(H)) of the hostmaterial 132. The triplet excitation energy level (T_(H)) of the hostmaterial 132 is higher than or equal to the triplet excitation energylevel (T_(G)) of the guest material 131. Therefore, the formulaΔE_(G)>ΔE_(H)≥S_(H)>T_(H)≥T_(G) (ΔE_(G) is greater than ΔE_(H), ΔE_(H)is greater than or equal to S_(H), S_(H) is higher than T_(H), and T_(H)is higher than or equal to T_(G)) is satisfied. Note that ΔT_(G) isequivalent to or slightly smaller than ΔE_(abs) in the case whereabsorption that relates to the absorption edge of the absorptionspectrum of the guest material 131 relates to transition between thesinglet ground state and the triplet excited state of the guest material131. Thus, in order to obtain ΔE_(G) larger than ΔE_(abs) by at least0.3 eV, the energy difference between S_(H) and T_(H) is preferablysmaller than the energy difference between ΔE_(G) and ΔE_(abs).Specifically, the energy difference between S_(H) and T_(H) ispreferably greater than 0 eV and less than or equal to 0.2 eV, morepreferably greater than 0 eV and less than or equal to 0.1 eV.

As an example of a material that has a small energy difference betweenthe singlet excitation energy level and the triplet excitation energylevel and is suitably used as the host material 132, a thermallyactivated delayed fluorescent (TADF) material can be given. Thethermally activated delayed fluorescent material has a small energydifference between the singlet excitation energy level and the tripletexcitation energy level and a function of converting triplet excitationenergy into singlet excitation energy by reverse intersystem crossing.Note that the host material 132 of one embodiment of the presentinvention need not necessarily have high reverse intersystem crossingefficiency from T_(H) to S_(H) and high luminescence quantum yield fromS_(H), whereby materials can be selected from a wide range of options.

In order to have a small difference between the singlet excitationenergy level and the triplet excitation energy level, the host material132 preferably includes a skeleton having a function of transportingholes (a hole-transport property) and a skeleton having a function oftransporting electrons (an electron-transport property). In this case,in the excited state of the host material 132, the skeleton having ahole-transport property includes the HOMO and the skeleton having anelectron-transport property includes the LUMO; thus, an overlap betweenthe HOMO and the LUMO is extremely small. That is, a donor-acceptorexcited state in a single molecule is easily formed, and the differencebetween the singlet excitation energy level and the triplet excitationenergy level is small. Note that in the host material 132, thedifference between the singlet excitation energy level (S_(H)) and thetriplet excitation energy level (T_(H)) is preferably greater than 0 eVand less than or equal to 0.2 eV.

Note that a molecular orbital refers to spatial distribution ofelectrons in a molecule, and can show the probability of finding ofelectrons. In addition, with the molecular orbital, electronconfiguration of the molecule (spatial distribution and energy ofelectrons) can be described in detail.

In the case where the host material 132 includes a skeleton having astrong donor property, a hole that has been injected to thelight-emitting layer 130 is easily injected to the host material 132 andeasily transported. In the case where the host material 132 includes askeleton having a strong acceptor property, an electron that has beeninjected to the light-emitting layer 130 is easily injected to the hostmaterial 132 and easily transported. Both holes and electrons arepreferably injected to the host material 132, in which case the excitedstate of the host material 132 is easily formed.

The shorter the emission wavelength of the guest material 131 is (thehigher light emission energy ΔE_(Em) is), the larger the energydifference (ΔE_(G)) between the LUMO level and the HOMO level of theguest material 131 is, and accordingly, larger energy is needed fordirectly and electrically exciting the guest material. However, in oneembodiment of the present invention, when the transition energy(ΔE_(abs)) calculated from the absorption edge of the absorptionspectrum of the guest material 131 is equivalent to or smaller thanΔE_(H), the guest material 131 can be excited with energy as small asΔE_(H), which is smaller than ΔE_(G), whereby the power consumption ofthe light-emitting element can be reduced. Therefore, the effect of thelight emission mechanism of one embodiment of the present invention isbrought to the fore in the case where the energy difference between thetransition energy (ΔE_(abs)) calculated from the absorption edge of theabsorption spectrum of the guest material 131 and the energy difference(ΔE_(G)) between the LUMO level and the HOMO level of the guest material131 is large (i.e., particularly in the case where the guest material isa blue light-emitting material).

As the transition energy (ΔE_(abs)) calculated from the absorption edgeof the absorption spectrum of the guest material 131 decreases, thelight emission energy (ΔE_(Em)) of the guest material 131 alsodecreases. In that case, light emission that needs high energy, such asblue light emission, is difficult to obtain. That is, when a differencebetween ΔE_(abs), and ΔE_(G) is too large, high-energy light emissionsuch as blue light emission is obtained with difficulty.

For these reasons, the energy difference (ΔE_(G)) between the LUMO leveland the HOMO level of the guest material 131 is preferably larger thanthe transition energy (ΔE_(abs)) calculated from the absorption edge ofthe absorption spectrum of the guest material 131 by 0.3 eV to 0.8 eVinclusive, more preferably by 0.4 eV to 0.8 eV inclusive, much morepreferably by 0.5 eV to 0.8 eV inclusive. Since the light emissionenergy (ΔE_(E)m) of the guest material 131 is equivalent to or smallerthan ΔE_(abs), the energy difference (ΔE_(G)) between the LUMO level andthe HOMO level of the guest material 131 is preferably larger than thelight emission energy (ΔE_(Em)) of the guest material 131 by 0.3 eV to0.8 eV inclusive, more preferably larger than that by 0.4 eV to 0.8 eVinclusive, much more preferably larger than that by 0.5 eV to 0.8 eVinclusive.

In addition, the guest material 131 serves as a hole trap in thelight-emitting layer 130 because of its HOMO level higher than the HOMOlevel of the host material 132. This is preferable because the carrierbalance in the light-emitting layer can be easily controlled, leading toa longer lifetime. However, when the HOMO level of the guest material131 is too high, the above-described ΔE_(B) becomes small. Therefore,the energy difference between the HOMO level of the guest material 131and the HOMO level of the host material 132 is preferably greater thanor equal to 0.05 eV and less than or equal to 0.4 eV. Furthermore, theenergy difference between the LUMO level of the guest material 131 andthe LUMO level of the host material 132 is preferably 0.05 eV or more,more preferably 0.1 eV or more, much more preferably 0.2 eV or more,which is suitable for easy injection of electron carriers to the hostmaterial 132.

Furthermore, since the energy difference (ΔE_(H)) between the LUMO leveland the HOMO level of the host material 132 is smaller than the energydifference (ΔE_(G)) between the LUMO level and the HOMO level of theguest material 131, an excited state formed by the host material 132 ismore energetically stable as an excited state formed by recombination ofcarriers (holes and electrons) injected to the light-emitting layer 130.Therefore, most of the excited states generated in the light-emittinglayer 130 by direct recombination of carriers exist as excited statesformed by the host material 132. Accordingly, the structure of oneembodiment of the present invention facilitates excitation energytransfer from the host material 132 to the guest material 131, leadingto lower driving voltage of the light-emitting element and higheremission efficiency.

According to the above-described relation between the LUMO level and theHOMO level, an oxidation potential of the guest material 131 ispreferably lower than an oxidation potential of the host material 132.Note that the oxidation potential and the reduction potential can bemeasured by cyclic voltammetry (CV).

When the light-emitting layer 130 has the above-described structure,light emission from the guest material 131 of the light-emitting layer130 can be obtained efficiently.

<Energy Transfer Mechanism>

Next, factors controlling the processes of intermolecular energytransfer between the host material 132 and the guest material 131 willbe described. As mechanisms of the intermolecular energy transfer, twomechanisms, i.e., Förster mechanism (dipole-dipole interaction) andDexter mechanism (electron exchange interaction), have been proposed.

<<Förster Mechanism>>

In Förster mechanism, energy transfer does not require direct contactbetween molecules and energy is transferred through a resonantphenomenon of dipolar oscillation between the host material 132 and theguest material 131. By the resonant phenomenon of dipolar oscillation,the host material 132 provides energy to the guest material 131, andthus, the host material 132 in an excited state is brought to a groundstate and the guest material 131 in a ground state is brought to anexcited state. Note that the rate constant k_(h*→g) of Förster mechanismis expressed by Formula (1).

$\begin{matrix}{k_{h^{*}\rightarrow g} = {\frac{9000c^{4}K^{2}{\varphi ln10}}{128\pi^{5}n^{4}N\; \tau \; R^{6}}{\int{\frac{{f_{h}^{\prime}(v)}{ɛ_{g}(v)}}{v^{4}}{dv}}}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$

In Formula (1), ν denotes a frequency, f′_(h)(ν) denotes a normalizedemission spectrum of the host material 132 (a fluorescence spectrum inenergy transfer from a singlet excited state, and a phosphorescencespectrum in energy transfer from a triplet excited state), ε_(g)(ν)denotes a molar absorption coefficient of the guest material 131, Ndenotes Avogadro's number, n denotes a refractive index of a medium, Rdenotes an intermolecular distance between the host material 132 and theguest material 131, τ denotes a measured lifetime of an excited state(fluorescence lifetime or phosphorescence lifetime), c denotes the speedof light, ϕ denotes a luminescence quantum yield (a fluorescence quantumyield in energy transfer from a singlet excited state, and aphosphorescence quantum yield in energy transfer from a triplet excitedstate), and K² denotes a coefficient (0 to 4) of orientation of atransition dipole moment between the host material 132 and the guestmaterial 131. Note that K²=2/3 in random orientation.

<<Dexter Mechanism>>

In Dexter mechanism, the host material 132 and the guest material 131are close to a contact effective range where their orbitals overlap, andthe host material 132 in an excited state and the guest material 131 ina ground state exchange their electrons, which leads to energy transfer.Note that the rate constant k_(h*→g) of Dexter mechanism is expressed byFormula (2).

$\begin{matrix}{k_{h^{*}\rightarrow g} = {\left( \frac{2\pi}{h} \right)K^{2}{\exp \left( {- \frac{2R}{L}} \right)}{\int{{f_{h}^{\prime}(v)}{ɛ_{g}^{\prime}(v)}{dv}}}}} & \left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack\end{matrix}$

In Formula (2), h denotes a Planck constant, K denotes a constant havingan energy dimension, ν denotes a frequency, f′_(h)(ν) denotes anormalized emission spectrum of the host material 132 (a fluorescencespectrum in energy transfer from a singlet excited state, and aphosphorescence spectrum in energy transfer from a triplet excitedstate), ε′_(g)(ν) denotes a normalized absorption spectrum of the guestmaterial 131, L denotes an effective molecular radius, and R denotes anintermolecular distance between the host material 132 and the guestmaterial 131.

Here, the efficiency of energy transfer from the host material 132 tothe guest material 131 (energy transfer efficiency ϕ_(ET)) is expressedby Formula (3). In the formula, k_(r) denotes a rate constant of alight-emission process (fluorescence in energy transfer from a singletexcited state, and phosphorescence in energy transfer from a tripletexcited state) of the host material 132, k_(n) denotes a rate constantof a non-light-emission process (thermal deactivation or intersystemcrossing) of the host material 132, and τ denotes a measured lifetime ofan excited state of the host material 132.

$\begin{matrix}{\varphi_{ET} = {\frac{k_{h^{*}\rightarrow g}}{k_{r} + k_{n} + k_{h^{*}\rightarrow g}} = \frac{k_{h^{*}\rightarrow g}}{\left( \frac{1}{\tau} \right) + k_{h^{*}\rightarrow g}}}} & \left\lbrack {{Formula}\mspace{14mu} 3} \right\rbrack\end{matrix}$

According to Formula (3), it is found that the energy transferefficiency ϕ_(ET) can be increased by increasing the rate constantk_(h*→g) of energy transfer so that another competing rate constantk_(r)+k_(n) (=1/τ) becomes relatively small.

<<Concept for Promoting Energy Transfer>>

In energy transfer by Förster mechanism, high energy transfer efficiencyϕ_(ET) is obtained when emission quantum yield ϕ (a fluorescence quantumyield in energy transfer from a singlet excited state, and aphosphorescence quantum yield in energy transfer from a triplet excitedstate) is high. Furthermore, it is preferable that the emission spectrum(the fluorescence spectrum in energy transfer from the singlet excitedstate) of the host material 132 largely overlap with the absorptionspectrum (absorption corresponding to the transition from the singletground state to the triplet excited state) of the guest material 131.Moreover, it is preferable that the molar absorption coefficient of theguest material 131 be also high. This means that the emission spectrumof the host material 132 overlaps with the absorption band of theabsorption spectrum of the guest material 131 that is on the longestwavelength side.

In energy transfer by Dexter mechanism, in order to make the rateconstant k_(h*→g) large, it is preferable that the emission spectrum (afluorescence spectrum in energy transfer from a singlet excited state,and a phosphorescence spectrum in energy transfer from a triplet excitedstate) of the host material 132 largely overlap with the absorptionspectrum (absorption corresponding to transition from a singlet groundstate to a triplet excited state) of the guest material 131. Therefore,the energy transfer efficiency can be optimized by making the emissionspectrum of the host material 132 overlap with the absorption band ofthe absorption spectrum of the guest material 131 that is on the longestwavelength side.

Structure Example 2 of Light-Emitting Element

Next, a light-emitting element having a structure different from thestructure illustrated in FIGS. 1A and 1B will be described below withreference to FIGS. 3A and 3B.

FIG. 3A is a schematic cross-sectional view of a light-emitting element152 of one embodiment of the present invention. In FIG. 3A, a portionhaving a function similar to that in FIG. 1A is represented by the samehatch pattern as in FIG. 1A and not especially denoted by a referencenumeral in some cases. In addition, common reference numerals are usedfor portions having similar functions, and a detailed description of theportions is omitted in some cases.

The light-emitting element 152 includes the pair of electrodes (theelectrode 101 and the electrode 102) and the EL layer 100 between thepair of electrodes. The EL layer 100 includes at least a light-emittinglayer 135.

FIG. 3B is a schematic cross-sectional view illustrating an example ofthe light-emitting layer 135 in FIG. 3A. The light-emitting layer 135 inFIG. 3B includes at least the guest material 131, the host material 132,and a host material 133.

In the light-emitting layer 135, the host material 132 or the hostmaterial 133 is present in the largest proportion by weight, and theguest material 131 is dispersed in the host material 132 and the hostmaterial 133.

<Light Emission Mechanism 2 of Light-Emitting Element>

Next, the light emission mechanism of the light-emitting layer 135 isdescribed.

Also in the light-emitting element 152 of one embodiment of the presentinvention, by recombination of electrons and holes injected from thepair of electrodes (the electrode 101 and the electrode 102), the guestmaterial 131 in the light-emitting layer 135 of the EL layer 100 isbrought into an excited state to provide light emission.

Note that light emission from the guest material 131 can be obtainedthrough the following two processes:

(α) direct recombination process; and

(β) energy transfer process.

Note that the direct recombination process (α) is not described herebecause it is similar to the direct recombination process in thedescription of the light emission mechanism of the light-emitting layer130.

<<(β) Energy Transfer Process>>

In order to describe the energy transfer process of the host material132, the host material 133, and the guest material 131, a schematicdiagram illustrating the correlation of energy levels is shown in FIG.4A. The following explain what terms and signs in FIG. 4A represent, andthe other terms and signs in FIG. 4A are similar to those in FIG. 2A.

Host (133): the host material 133;

S_(A): an S1 level of the host material 133; and

T_(A): a T1 level of the host material 133.

In the case where carriers are recombined in the host material 132 andthe singlet excited state and the triplet excited state of the hostmaterial 132 are formed, as shown in Route E₁ and Route E₂ in FIG. 4A,both of the singlet excitation energy and the triplet excitation energyof the host material 132 are transferred from the singlet excitationenergy level (S_(H)) and the triplet excitation energy level (T_(H)) ofthe host material 132 to the triplet excitation energy level (T) of theguest material 131, and the guest material 131 is brought into a tripletexcited state. Phosphorescence is obtained from the guest material 131in the triplet excited state.

Note that in order to transfer excitation energy from the host material132 to the guest material 131 efficiently, the triplet excitation energylevel (T_(A)) of the host material 133 is preferably higher than thetriplet excitation energy level (T_(H)) of the host material 132. Thus,quenching of the triplet excitation energy of the host material 132 isless likely to occur, which causes efficient energy transfer to theguest material 131.

When the HOMO level of the guest material 131 is higher than the HOMOlevel of the host material 132 as shown in an energy band diagram inFIG. 4B, it is preferable that the energy difference (ΔE_(G)) betweenthe LUMO level and the HOMO level of the guest material 131 be largerthan the energy difference (ΔE_(H)) between the LUMO level and the HOMOlevel of the host material 132 and that ΔE_(H) be larger than the energydifference (ΔE_(B)) between the LUMO level of the host material 132 andthe HOMO level of the guest material 131, as described in Light emissionmechanism 1 of light-emitting element.

It is preferable that the LUMO level of the host material 133 be higherthan the LUMO level of the host material 132 and that the HOMO level ofthe host material 133 be lower than the HOMO level of the guest material131. That is, the energy difference between the LUMO level and the HOMOlevel of the host material 133 is larger than the energy difference(ΔE_(B)) between the LUMO level of the host material 132 and the HOMOlevel of the guest material 131. Thus, the reaction for forming anexciplex by the host material 133 and the host material 132 and thereaction for forming an exciplex by the host material 133 and the guestmaterial 131 can be inhibited. In FIG. 4B, “Host (133)” represents thehost material 133, and the other terms and signs are similar to those inFIG. 2B.

Note that the difference between the LUMO level of the host material 133and the LUMO level of the host material 132 and the difference betweenthe HOMO level of the host material 133 and the HOMO level of the guestmaterial 131 are each preferably greater than or equal to 0.1 eV, morepreferably greater than or equal to 0.2 eV. The energy difference issuitable because electron carriers and hole carriers injected from thepair of electrodes (the electrode 101 and the electrode 102) are easilyinjected to the host material 132 and the guest material 131,respectively.

Note that the LUMO level of the host material 133 may be either higheror lower than the LUMO level of the guest material 131, and the HOMOlevel of the host material 133 may be either higher or lower than theHOMO level of the host material 132.

Furthermore, the energy difference between the LUMO level and the HOMOlevel of the host material 133 is preferably larger than the energydifference (ΔE_(H)) between the LUMO level and the HOMO level of thehost material 132. In that case, since the energy difference (ΔE_(H))between the LUMO level and the HOMO level of the host material 132 issmaller than the energy difference (ΔE_(G)) between the LUMO level andthe HOMO level of the guest material 131, as an excited state formed byrecombination of carriers (holes and electrons) injected to thelight-emitting layer 135, an excited state formed by the host material132 is more energetically stable than an excited state formed by thehost material 133 and an excited state formed by the guest material 131.Therefore, most of the excited states generated in the light-emittinglayer 135 by recombination of carriers exist as excited states formed bythe host material 132. Thus, in the light-emitting layer 135, excitationenergy transfer from an excited state of the host material 132 to theguest material 131 occurs easily as in the structure of thelight-emitting layer 130, so that the light-emitting element 152 can bedriven with low driving voltage and high emission efficiency.

Even in the case where holes and electrons are recombined in the hostmaterial 133 and an excited state is formed by the host material 133,excitation energy of the host material 133 can be immediatelytransferred to the host material 132 when the energy difference betweenthe LUMO level and the HOMO level of the host material 133 is largerthan the energy difference between the LUMO level and the HOMO level ofthe host material 132. Then, the excitation energy is transferred to theguest material 131 through a process similar to that in the descriptionof the light emission mechanism of the light-emitting layer 130, wherebylight emission from the guest material 131 can be obtained. Note thatwhen the possibility that holes and electrons are recombined also in thehost material 133 is taken into consideration, the host material 133 ispreferably a material having a small energy difference between thesinglet excitation energy level and the triplet excitation energy level,particularly preferably a thermally activated delayed fluorescentmaterial, like the host material 132.

In order to obtain light emission from the guest material 131efficiently, it is preferable that the singlet excitation energy level(S_(A)) of the host material 133 be higher than or equal to the singletexcitation energy level (S_(H)) of the host material 132 and that thetriplet excitation energy level (T_(A)) of the host material 133 behigher than or equal to the triplet excitation energy level (T_(H)) ofthe host material 132.

According to the above-described relations between the LUMO levels andthe HOMO levels, it is preferable that a reduction potential of the hostmaterial 133 be lower than a reduction potential of the host material132 and that an oxidation potential of the host material 133 be higherthan the oxidation potential of the guest material 131.

In the case where the combination of the host material 132 and the hostmaterial 133 is a combination of a material having a function oftransporting holes and a material having a function of transportingelectrons, the carrier balance can be easily controlled depending on themixture ratio. Specifically, the ratio of the material having a functionof transporting holes to the material having a function of transportingelectrons is preferably within a range of 1:9 to 9:1 (weight ratio).Since the carrier balance can be easily controlled with the structure, acarrier recombination region can also be controlled easily.

When the light-emitting layer 135 has the above-described structure,light emission from the guest material 131 of the light-emitting layer135 can be obtained efficiently.

<Material>

Next, components of a light-emitting element of one embodiment of thepresent invention are described in detail below.

<<Light-Emitting Layer>>

In the light-emitting layer 130 and the light-emitting layer 135, theweight percentage of the host material 132 is higher than that of atleast the guest material 131, and the guest material 131 (thephosphorescent material) is dispersed in the host material 132.

<<Host Material 132>>

The energy difference between the S1 level and the T1 level of the hostmaterial 132 is preferably small, and specifically, greater than 0 eVand less than or equal to 0.2 eV.

The host material 132 preferably includes a skeleton having ahole-transport property and a skeleton having an electron-transportproperty. Alternatively, the host material 132 preferably includes aπ-electron deficient heteroaromatic ring skeleton and one of aπ-electron rich heteroaromatic ring skeleton and an aromatic amineskeleton. Thus, a donor-acceptor excited state is easily formed in amolecule. Furthermore, to increase both the donor property and theacceptor property in the molecule of the host material 132, a structurewhere the skeleton having an electron-transport property and theskeleton having a hole-transport property are directly bonded to eachother is preferably included. Alternatively, it is preferable that astructure where a π-electron deficient heteroaromatic ring skeleton isdirectly bonded to one of a π-electron rich heteroaromatic ring skeletonand an aromatic amine skeleton be included. By increasing both the donorproperty and the acceptor property in the molecule, an overlap between aregion where the HOMO is distributed and a region where the LUMO isdistributed in the host material 132 can be small, and the energydifference between the singlet excitation energy level and the tripletexcitation energy level of the host material 132 can be small. Moreover,the triplet excitation energy level of the host material 132 can be kepthigh.

As an example of the material in which the energy difference between thetriplet excitation energy level and the singlet excitation energy levelis small, a thermally activated delayed fluorescent material can begiven. Note that a thermally activated delayed fluorescent material hasa function of converting triplet excited energy into singlet excitedenergy by reverse intersystem crossing because of having a smalldifference between the triplet excited energy level and the singletexcited energy level. Thus, the TADF material can up-convert a tripletexcited state into a singlet excited state (i.e., reverse intersystemcrossing is possible) using a little thermal energy and efficientlyexhibit light emission (fluorescence) from the singlet excited state.The TADF material is efficiently obtained under the condition where thedifference between the triplet excited energy level and the singletexcited energy level is preferably larger than 0 eV and smaller than orequal to 0.2 eV, more preferably larger than 0 eV and smaller than orequal to 0.1 eV.

In the case where the TADF material is composed of one kind of material,any of the following materials can be used, for example.

First, a fullerene, a derivative thereof, an acridine derivative such asproflavine, eosin, and the like can be given. Furthermore, ametal-containing porphyrin, such as a porphyrin containing magnesium(Mg), zinc (Zn), cadmium (Cd), tin (Sn), platinum (Pt), indium (In), orpalladium (Pd), can be given. Examples of the metal-containing porphyrininclude a protoporphyrin-tin fluoride complex (SnF₂(Proto IX)), amesoporphyrin-tin fluoride complex (SnF₂(Meso IX)), ahematoporphyrin-tin fluoride complex (SnF₂(Hemato IX)), a coproporphyrintetramethyl ester-tin fluoride complex (SnF₂(Copro III-4Me)), anoctaethylporphyrin-tin fluoride complex (SnF₂(OEP)), anetioporphyrin-tin fluoride complex (SnF₂(Etio I)), and anoctaethylporphyrin-platinum chloride complex (PtCl₂OEP).

As the TADF material composed of one kind of material, a heterocycliccompound including a π-electron rich heteroaromatic ring and aπ-electron deficient heteroaromatic ring can also be used. Specifically,2-(biphenyl-4-yl)-4,6-bis(12-phenylindolo[2,3-a]carbazol-11-yl)-1,3,5-triazine(abbreviation: PIC-TRZ),2-{4-[3-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}-4,6-diphenyl-1,3,5-triazine(abbreviation: PCCzPTzn),2-[4-(10H-phenoxazin-10-yl)phenyl]-4,6-diphenyl-1,3,5-triazine(abbreviation: PXZ-TRZ),3-[4-(5-phenyl-5,10-dihydrophenazin-10-yl)phenyl]-4,5-diphenyl-1,2,4-triazole(abbreviation: PPZ-3TPT),3-(9,9-dimethyl-9H-acridin-10-yl)-9H-xanthen-9-one (abbreviation:ACRXTN), bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone(abbreviation: DMAC-DPS), or10-phenyl-10H,10′H-spiro[acridin-9,9′-anthracen]-10′-one (abbreviation:ACRSA) can be used. The heterocyclic compound is preferably used becauseof having the π-electron rich heteroaromatic ring and the π-electrondeficient heteroaromatic ring, for which the electron-transport propertyand the hole-transport property are high. Among skeletons having theπ-electron deficient heteroaromatic ring, a diazine skeleton (apyrimidine skeleton, a pyrazine skeleton, or a pyridazine skeleton) anda triazine skeleton have high stability and high reliability and areparticularly preferable. Among skeletons having the π-electron richheteroaromatic ring, an acridine skeleton, a phenoxazine skeleton, aphenothiazine skeleton, a furan skeleton, a thiophene skeleton, and apyrrole skeleton have high stability and high reliability; therefore, atleast one of these skeletons are preferably included. As the furanskeleton, a dibenzofuran skeleton is preferable. As the thiopheneskeleton, a dibenzothiophene skeleton is preferable. As the pyrroleskeleton, an indole skeleton, a carbazole skeleton, or a9-phenyl-3,3′-bi-9H-carbazole skeleton is particularly preferred. Notethat a substance in which the π-electron rich heteroaromatic ring isdirectly bonded to the π-electron deficient heteroaromatic ring isparticularly preferably used because the donor property of theπ-electron rich heteroaromatic ring and the acceptor property of theπ-electron deficient heteroaromatic ring are both increased and thedifference between the level of the singlet excited state and the levelof the triplet excited state becomes small. Note that an aromatic ringto which an electron-withdrawing group such as a cyano group is bondedmay be used instead of the π-electron deficient heteroaromatic ring.

Among skeletons having the π-electron deficient heteroaromatic ring, acondensed heterocyclic skeleton having a diazine skeleton is preferablebecause of having higher stability and higher reliability, and abenzofuropyrimidine skeleton and a benzothienopyrimidine skeleton areparticularly preferable because of having a higher acceptor property. Asthe benzofuropyrimidine skeleton, for example, abenzofuro[3,2-d]pyrimidine skeleton is given. As thebenzothienopyrimidine skeleton, for example, abenzothieno[3,2-d]pyrimidine skeleton is given.

Among skeletons having the π-electron rich heteroaromatic ring, abicarbazole skeleton is preferable because of having high excitationenergy, high stability, and high reliability. As the bicarbazoleskeleton, for example, a bicarbazole skeleton in which any of the 2- to4-positions of a carbazolyl group is bonded to any of the 2- to4-positions of another carbazolyl group is particularly preferablebecause of having a high donor property. As such a bicarbazole skeleton,for example, 2,2′-bi-9H-carbazole skeleton, 3,3′-bi-9H-carbazoleskeleton, 4,4′-bi-9H-carbazole skeleton, 2,3′-bi-9H-carbazole skeleton,2,4′-bi-9H-carbazole skeleton, 3,4′-bi-9H-carbazole skeleton, and thelike are given.

In view of increasing a band gap and a triplet excitation energy, acompound in which the 9-position of one of the carbazolyl groups in thebicarbazole skeleton is directly bonded to the benzofuropyrimidineskeleton or the benzothienopyrimidine skeleton is preferable. In thecase where the bicarbazole skeleton is directly bonded to thebenzofuropyrimidine skeleton or the benzothienopyrimidine skeleton, arelatively low molecular compound is formed, and therefore, a structurethat is suitable for vacuum evaporation (a structure that can be formedby vacuum evaporation at a relatively low temperature) is obtained,which is preferable. In general, a lower molecular weight tends toreduce heat resistance after film formation. However, because of highrigidity of the benzofuropyrimidine skeleton, the benzothienopyrimidineskeleton, and the bicarbazole skeleton, a compound including theskeleton can have sufficient heat resistance even with a relatively lowmolecular weight. The structure is preferable because a band gap and anexcitation energy level are increased.

In the case where the bicarbazole skeleton is bonded to thebenzofuropyrimidine skeleton or the benzothienopyrimidine skeletonthrough an arylene group having 6 to 25 carbon atoms, preferably 6 to 13carbon atoms, the band gap is kept wide and the triplet excitationenergy can be kept high. Moreover, a relatively low molecular compoundis formed, and therefore, a structure that is suitable for vacuumevaporation (a structure that can be formed by vacuum evaporation at arelatively low temperature) is obtained.

In the case where a bicarbazole skeleton is bonded, directly or throughan arylene group, to a benzofuro[3,2-d]pyrimidine skeleton or abenzothieno[3,2-d]pyrimidine skeleton, preferably the 4-position of thebenzofuro[3,2-d]pyrimidine skeleton or the benzothieno[3,2-d]pyrimidineskeleton in a compound, the compound has a high carrier-transportproperty. Accordingly, a light-emitting element using the compound canbe driven at a low voltage.

Compound Example 1

The above-described compound that is preferably used in a light-emittingelement of one embodiment of the present invention is a compoundrepresented by General Formula (G0).

In General Formula (G0), A represents a substituted or unsubstitutedbenzofuropyrimidine skeleton or a substituted or unsubstitutedbenzothienopyrimidine skeleton. In the case where thebenzofuropyrimidine skeleton or the benzothienopyrimidine skeleton has asubstituent, as the substituent, an alkyl group having 1 to 6 carbonatoms, a cycloalkyl group having 3 to 7 carbon atoms, or a substitutedor unsubstituted aryl group having 6 to 13 carbon atoms can also beselected. Specific examples of the alkyl group having 1 to 6 carbonatoms include a methyl group, an ethyl group, a propyl group, anisopropyl group, a butyl group, an isobutyl group, a tert-butyl group,an n-hexyl group, and the like. Specific examples of a cycloalkyl grouphaving 3 to 7 carbon atoms include a cyclopropyl group, a cyclobutylgroup, a cyclopentyl group, a cyclohexyl group, and the like. Specificexamples of the aryl group having 6 to 13 carbon atoms include a phenylgroup, a naphthyl group, a biphenyl group, a fluorenyl group, and thelike.

Further, each of R¹ to R¹⁵ independently represents any of hydrogen, asubstituted or unsubstituted alkyl group having 1 to 6 carbon atoms, asubstituted or unsubstituted cycloalkyl group having 3 to 7 carbonatoms, and a substituted or unsubstituted aryl group having 6 to 13carbon atoms. Specific examples of the alkyl group having 1 to 6 carbonatoms include a methyl group, an ethyl group, a propyl group, anisopropyl group, a butyl group, an isobutyl group, a tert-butyl group,an n-hexyl group, and the like. Specific examples of a cycloalkyl grouphaving 3 to 7 carbon atoms include a cyclopropyl group, a cyclobutylgroup, a cyclopentyl group, a cyclohexyl group, and the like. Specificexamples of the aryl group having 6 to 13 carbon atoms include a phenylgroup, a naphthyl group, a biphenyl group, a fluorenyl group, and thelike. The above alkyl group, cycloalkyl group, and aryl group mayinclude one or more substituents, and the substituents may be bonded toeach other to form a ring. As the substituent, an alkyl group having 1to 6 carbon atoms, a cycloalkyl group having 3 to 7 carbon atoms, or anaryl group having 6 to 13 carbon atoms can also be selected. Specificexamples of the alkyl group having 1 to 6 carbon atoms include a methylgroup, an ethyl group, a propyl group, an isopropyl group, a butylgroup, an isobutyl group, a tert-butyl group, an n-hexyl group, and thelike. Specific examples of a cycloalkyl group having 3 to 7 carbon atomsinclude a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, acyclohexyl group, and the like. Specific examples of the aryl grouphaving 6 to 13 carbon atoms include a phenyl group, a naphthyl group, abiphenyl group, a fluorenyl group, and the like.

Further, Ar¹ represents an arylene group having 6 to 25 carbon atoms ora single bond. The arylene group may include one or more substituentsand the substituents may be bonded to each other to form a ring. Forexample, a carbon atom at the 9-position in a fluorenyl group has twophenyl groups as substituents and the phenyl groups are bonded to form aspirofluorene skeleton. Specific examples of the arylene group having 6to 25 carbon atoms include a phenylene group, a naphthylene group, abiphenyldiyl group, a fluorenediyl group, and the like. In the casewhere the arylene group has a substituent, as the substituent, an alkylgroup having 1 to 6 carbon atoms, a cycloalkyl group having 3 to 7carbon atoms, or an aryl group having 6 to 13 carbon atoms can also beselected. Specific examples of the alkyl group having 1 to 6 carbonatoms include a methyl group, an ethyl group, a propyl group, anisopropyl group, a butyl group, an isobutyl group, a tert-butyl group,an n-hexyl group, and the like. Specific examples of a cycloalkyl grouphaving 3 to 7 carbon atoms include a cyclopropyl group, a cyclobutylgroup, a cyclopentyl group, a cyclohexyl group, and the like. Specificexamples of the aryl group having 6 to 13 carbon atoms include a phenylgroup, a naphthyl group, a biphenyl group, a fluorenyl group, and thelike.

In the compound represented by General Formula (G0), thebenzofuropyrimidine skeleton is preferably a benzofuro[3,2-d]pyrimidineskeleton, and the benzothienopyrimidine skeleton is preferably abenzothieno[3,2-d]pyrimidine skeleton.

The compound represented by General Formula (G0) in which the 9-positionof one of the carbazolyl groups in the bicarbazole skeleton is bonded,directly or through the arylene group, to the 4-position of thebenzofuro[3,2-d]pyrimidine skeleton or the benzothieno[3,2-d]pyrimidineskeleton has a high donor property, a high acceptor property, and a wideband gap, and therefore can suitably be used in a light-emitting elementthat emits light with high energy such as blue light, which ispreferable. The above-described compound is a compound represented byGeneral Formula (G1).

In General Formula (G1), Q represents oxygen or sulfur.

Further, each of R¹ to R²⁰ independently represents any of hydrogen, asubstituted or unsubstituted alkyl group having 1 to 6 carbon atoms, asubstituted or unsubstituted cycloalkyl group having 3 to 7 carbonatoms, and a substituted or unsubstituted aryl group having 6 to 13carbon atom. Specific examples of the alkyl group having 1 to 6 carbonatoms include a methyl group, an ethyl group, a propyl group, anisopropyl group, a butyl group, an isobutyl group, a tert-butyl group,an n-hexyl group, and the like. Specific examples of a cycloalkyl grouphaving 3 to 7 carbon atoms include a cyclopropyl group, a cyclobutylgroup, a cyclopentyl group, a cyclohexyl group, and the like. Specificexamples of the aryl group having 6 to 13 carbon atoms include a phenylgroup, a naphthyl group, a biphenyl group, a fluorenyl group, and thelike. The above alkyl group, cycloalkyl group, and aryl group mayinclude one or more substituents, and the substituents may be bonded toeach other to form a ring. As the substituent, an alkyl group having 1to 6 carbon atoms, a cycloalkyl group having 3 to 7 carbon atoms, or anaryl group having 6 to 13 carbon atoms can also be selected. Specificexamples of the alkyl group having 1 to 6 carbon atoms include a methylgroup, an ethyl group, a propyl group, an isopropyl group, a butylgroup, an isobutyl group, a tert-butyl group, an n-hexyl group, and thelike. Specific examples of a cycloalkyl group having 3 to 7 carbon atomsinclude a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, acyclohexyl group, and the like. Specific examples of the aryl grouphaving 6 to 13 carbon atoms include a phenyl group, a naphthyl group, abiphenyl group, a fluorenyl group, and the like.

Further, Ar¹ represents an arylene group having 6 to 25 carbon atoms ora single bond. The arylene group may include one or more substituentsand the substituents may be bonded to each other to form a ring. Forexample, a carbon atom at the 9-position in a fluorenyl group has twophenyl groups as substituents and the phenyl groups are bonded to form aspirofluorene skeleton. Specific examples of the arylene group having 6to 25 carbon atoms include a phenylene group, a naphthylene group, abiphenyldiyl group, a fluorenediyl group, and the like. In the casewhere the arylene group has a substituent, as the substituent, an alkylgroup having 1 to 6 carbon atoms, a cycloalkyl group having 3 to 7carbon atoms, or an aryl group having 6 to 13 carbon atoms can also beselected. Specific examples of the alkyl group having 1 to 6 carbonatoms include a methyl group, an ethyl group, a propyl group, anisopropyl group, a butyl group, an isobutyl group, a tert-butyl group,an n-hexyl group, and the like. Specific examples of a cycloalkyl grouphaving 3 to 7 carbon atoms include a cyclopropyl group, a cyclobutylgroup, a cyclopentyl group, a cyclohexyl group, and the like. Specificexamples of the aryl group having 6 to 13 carbon atoms include a phenylgroup, a naphthyl group, a biphenyl group, a fluorenyl group, and thelike.

The compound represented by General Formula (G1) in which thebicarbazole skeleton is a 3,3′-bi-9H-carbazole skeleton and the9-position of one of the carbazolyl groups in the bicarbazole skeletonis bonded, directly or through the arylene group, to the 4-position ofthe benzofuro[3,2-d]pyrimidine skeleton or thebenzothieno[3,2-d]pyrimidine skeleton has a high carrier-transportproperty and a light-emitting element including the compound can bedriven at a low voltage, which is preferable. The above-describedcompound is a compound represented by General Formula (G2).

In General Formula (G2), Q represents oxygen or sulfur.

Further, each of R¹ to R²⁰ independently represents any of hydrogen, asubstituted or unsubstituted alkyl group having 1 to 6 carbon atoms, asubstituted or unsubstituted cycloalkyl group having 3 to 7 carbonatoms, and a substituted or unsubstituted aryl group having 6 to 13carbon atom. Specific examples of the alkyl group having 1 to 6 carbonatoms include a methyl group, an ethyl group, a propyl group, anisopropyl group, a butyl group, an isobutyl group, a tert-butyl group,an n-hexyl group, and the like. Specific examples of a cycloalkyl grouphaving 3 to 7 carbon atoms include a cyclopropyl group, a cyclobutylgroup, a cyclopentyl group, a cyclohexyl group, and the like. Specificexamples of the aryl group having 6 to 13 carbon atoms include a phenylgroup, a naphthyl group, a biphenyl group, a fluorenyl group, and thelike. The above alkyl group, cycloalkyl group, and aryl group mayinclude one or more substituents, and the substituents may be bonded toeach other to form a ring. As the substituent, an alkyl group having 1to 6 carbon atoms, a cycloalkyl group having 3 to 7 carbon atoms, or anaryl group having 6 to 13 carbon atoms can also be selected. Specificexamples of the alkyl group having 1 to 6 carbon atoms include a methylgroup, an ethyl group, a propyl group, an isopropyl group, a butylgroup, an isobutyl group, a tert-butyl group, an n-hexyl group, and thelike. Specific examples of a cycloalkyl group having 3 to 7 carbon atomsinclude a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, acyclohexyl group, and the like. Specific examples of the aryl grouphaving 6 to 13 carbon atoms include a phenyl group, a naphthyl group, abiphenyl group, a fluorenyl group, and the like.

Furthermore, Ar¹ represents an arylene group having 6 to 25 carbon atomsor a single bond. The arylene group may include one or more substituentsand the substituents may be bonded to each other to form a ring. Forexample, a carbon atom at the 9-position in a fluorenyl group has twophenyl groups as substituents and the phenyl groups are bonded to form aspirofluorene skeleton. Specific examples of the arylene group having 6to 13 carbon atoms include a phenylene group, a naphthylene group, abiphenyldiyl group, a fluorenediyl group, and the like. In the casewhere the arylene group has a substituent, as the substituent, an alkylgroup having 1 to 6 carbon atoms, a cycloalkyl group having 3 to 7carbon atoms, or an aryl group having 6 to 13 carbon atoms can also beselected. Specific examples of the alkyl group having 1 to 6 carbonatoms include a methyl group, an ethyl group, a propyl group, anisopropyl group, a butyl group, an isobutyl group, a tert-butyl group,an n-hexyl group, and the like. Specific examples of a cycloalkyl grouphaving 3 to 7 carbon atoms include a cyclopropyl group, a cyclobutylgroup, a cyclopentyl group, a cyclohexyl group, and the like. Specificexamples of the aryl group having 6 to 13 carbon atoms include a phenylgroup, a naphthyl group, a biphenyl group, a fluorenyl group, and thelike.

In the case where the bicarbazole skeleton is directly bonded to thebenzofuropyrimidine skeleton or the benzothienopyrimidine skeleton inthe compound represented by General Formula (G1) or (G2), the compoundhas a wider bandgap and can be synthesized with higher purity, which ispreferable. Because the compound has an excellent carrier-transportproperty, a light-emitting element including the compound can be drivenat a low voltage, which is preferable.

In the case where each of R¹ to R¹⁴ and R¹⁶ to R²⁰ represents hydrogenin General Formula (G1) or (G2), the compound is advantageous in termsof easiness of synthesis and material cost and has a relatively lowmolecular weight to be suitable for vacuum evaporation, which isparticularly preferable. The compound is a compound represented byGeneral Formula (G3) or (G4).

In General Formula (G3), Q represents oxygen or sulfur.

Further, R¹⁵ represents any of hydrogen, a substituted or unsubstitutedalkyl group having 1 to 6 carbon atoms, a substituted or unsubstitutedcycloalkyl group having 3 to 7 carbon atoms, and a substituted orunsubstituted aryl group having 6 to 13 carbon atoms. Specific examplesof the alkyl group having 1 to 6 carbon atoms include a methyl group, anethyl group, a propyl group, an isopropyl group, a butyl group, anisobutyl group, a tert-butyl group, an n-hexyl group, and the like.Specific examples of a cycloalkyl group having 3 to 7 carbon atomsinclude a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, acyclohexyl group, and the like. Specific examples of the aryl grouphaving 6 to 13 carbon atoms include a phenyl group, a naphthyl group, abiphenyl group, a fluorenyl group, and the like. The above alkyl group,cycloalkyl group, and aryl group may include one or more substituents,and the substituents may be bonded to each other to form a ring. As thesubstituent, an alkyl group having 1 to 6 carbon atoms, a cycloalkylgroup having 3 to 7 carbon atoms, or an aryl group having 6 to 13 carbonatoms can also be selected. Specific examples of the alkyl group having1 to 6 carbon atoms include a methyl group, an ethyl group, a propylgroup, an isopropyl group, a butyl group, an isobutyl group, atert-butyl group, an n-hexyl group, and the like. Specific examples of acycloalkyl group having 3 to 7 carbon atoms include a cyclopropyl group,a cyclobutyl group, a cyclopentyl group, a cyclohexyl group, and thelike. Specific examples of the aryl group having 6 to 13 carbon atomsinclude a phenyl group, a naphthyl group, a biphenyl group, a fluorenylgroup, and the like.

Furthermore, Ar¹ represents an arylene group having 6 to 25 carbon atomsor a single bond. The arylene group may include one or more substituentsand the substituents may be bonded to each other to form a ring. Forexample, a carbon atom at the 9-position in a fluorenyl group has twophenyl groups as substituents and the phenyl groups are bonded to form aspirofluorene skeleton. Specific examples of the arylene group having 6to 25 carbon atoms include a phenylene group, a naphthylene group, abiphenyldiyl group, a fluorenediyl group, and the like. In the casewhere the arylene group has a substituent, as the substituent, an alkylgroup having 1 to 6 carbon atoms, a cycloalkyl group having 3 to 7carbon atoms, or an aryl group having 6 to 13 carbon atoms can also beselected. Specific examples of the alkyl group having 1 to 6 carbonatoms include a methyl group, an ethyl group, a propyl group, anisopropyl group, a butyl group, an isobutyl group, a tert-butyl group,an n-hexyl group, and the like. Specific examples of a cycloalkyl grouphaving 3 to 7 carbon atoms include a cyclopropyl group, a cyclobutylgroup, a cyclopentyl group, a cyclohexyl group, and the like. Specificexamples of the aryl group having 6 to 13 carbon atoms include a phenylgroup, a naphthyl group, a biphenyl group, a fluorenyl group, and thelike.

In General Formula (G4), Q represents oxygen or sulfur.

Further, R¹ represents any of hydrogen, a substituted or unsubstitutedalkyl group having 1 to 6 carbon atoms, a substituted or unsubstitutedcycloalkyl group having 3 to 7 carbon atoms, and a substituted orunsubstituted aryl group having 6 to 13 carbon atom. Specific examplesof the alkyl group having 1 to 6 carbon atoms include a methyl group, anethyl group, a propyl group, an isopropyl group, a butyl group, anisobutyl group, a tert-butyl group, an n-hexyl group, and the like.Specific examples of a cycloalkyl group having 3 to 7 carbon atomsinclude a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, acyclohexyl group, and the like. Specific examples of the aryl grouphaving 6 to 13 carbon atoms include a phenyl group, a naphthyl group, abiphenyl group, a fluorenyl group, and the like. The above alkyl group,cycloalkyl group, and aryl group may include one or more substituents,and the substituents may be bonded to each other to form a ring. As thesubstituent, an alkyl group having 1 to 6 carbon atoms, a cycloalkylgroup having 3 to 7 carbon atoms, or an aryl group having 6 to 13 carbonatoms can also be selected. Specific examples of the alkyl group having1 to 6 carbon atoms include a methyl group, an ethyl group, a propylgroup, an isopropyl group, a butyl group, an isobutyl group, atert-butyl group, an n-hexyl group, and the like. Specific examples of acycloalkyl group having 3 to 7 carbon atoms include a cyclopropyl group,a cyclobutyl group, a cyclopentyl group, a cyclohexyl group, and thelike. Specific examples of the aryl group having 6 to 13 carbon atomsinclude a phenyl group, a naphthyl group, a biphenyl group, a fluorenylgroup, and the like.

Furthermore, Ar¹ represents an arylene group having 6 to 25 carbon atomsor a single bond. The arylene group may include one or more substituentsand the substituents may be bonded to each other to form a ring. Forexample, a carbon atom at the 9-position in a fluorenyl group has twophenyl groups as substituents and the phenyl groups are bonded to form aspirofluorene skeleton. Specific examples of the arylene group having 6to 25 carbon atoms include a phenylene group, a naphthylene group, abiphenyldiyl group, a fluorenediyl group, and the like. In the casewhere the arylene group has a substituent, as the substituent, an alkylgroup having 1 to 6 carbon atoms, a cycloalkyl group having 3 to 7carbon atoms, or an aryl group having 6 to 13 carbon atoms can also beselected. Specific examples of the alkyl group having 1 to 6 carbonatoms include a methyl group, an ethyl group, a propyl group, anisopropyl group, a butyl group, an isobutyl group, a tert-butyl group,an n-hexyl group, and the like. Specific examples of a cycloalkyl grouphaving 3 to 7 carbon atoms include a cyclopropyl group, a cyclobutylgroup, a cyclopentyl group, a cyclohexyl group, and the like. Specificexamples of the aryl group having 6 to 13 carbon atoms include a phenylgroup, a naphthyl group, a biphenyl group, a fluorenyl group, and thelike.

As the benzofuropyrimidine skeleton or the benzothienopyrimidineskeleton represented by A in General Formula (G0), any of structuresrepresented by Structural Formulae (Ht-1) to (Ht-24) can be used, forexample. Note that a structure that can be used as A is not limited tothese.

In Structural Formulae (Ht-1) to (Ht-24), each of R⁶ to R²⁰independently represents any of hydrogen, a substituted or unsubstitutedalkyl group having 1 to 6 carbon atoms, a substituted or unsubstitutedcycloalkyl group having 3 to 7 carbon atoms, and a substituted orunsubstituted aryl group having 6 to 13 carbon atoms. Specific examplesof the alkyl group having 1 to 6 carbon atoms include a methyl group, anethyl group, a propyl group, an isopropyl group, a butyl group, anisobutyl group, a tert-butyl group, an n-hexyl group, and the like.Specific examples of a cycloalkyl group having 3 to 7 carbon atomsinclude a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, acyclohexyl group, and the like. Specific examples of the aryl grouphaving 6 to 13 carbon atoms include a phenyl group, a naphthyl group, abiphenyl group, a fluorenyl group, and the like. The above alkyl group,cycloalkyl group, and aryl group may include one or more substituents,and the substituents may be bonded to each other to form a ring. As thesubstituent, an alkyl group having 1 to 6 carbon atoms, a cycloalkylgroup having 3 to 7 carbon atoms, or an aryl group having 6 to 13 carbonatoms can also be selected. Specific examples of the alkyl group having1 to 6 carbon atoms include a methyl group, an ethyl group, a propylgroup, an isopropyl group, a butyl group, an isobutyl group, atert-butyl group, an n-hexyl group, and the like. Specific examples of acycloalkyl group having 3 to 7 carbon atoms include a cyclopropyl group,a cyclobutyl group, a cyclopentyl group, a cyclohexyl group, and thelike. Specific examples of the aryl group having 6 to 13 carbon atomsinclude a phenyl group, a naphthyl group, a biphenyl group, a fluorenylgroup, and the like.

As a structure that can be used as the bicarbazole skeleton in GeneralFormulae (G0) and (G1), any of structures represented by StructuralFormulae (Cz-1) to (Cz-9) can be used, for example. Note that thestructure that can be used as the bicarbazole skeleton is not limited tothese.

In Structural Formulae (Cz-1) to (Cz-9), each of R¹ to R¹⁵ independentlyrepresents any of hydrogen, a substituted or unsubstituted alkyl grouphaving 1 to 6 carbon atoms, a substituted or unsubstituted cycloalkylgroup having 3 to 7 carbon atoms, and a substituted or unsubstitutedaryl group having 6 to 13 carbon atoms. Specific examples of the alkylgroup having 1 to 6 carbon atoms include a methyl group, an ethyl group,a propyl group, an isopropyl group, a butyl group, an isobutyl group, atert-butyl group, an n-hexyl group, and the like. Specific examples of acycloalkyl group having 3 to 7 carbon atoms include a cyclopropyl group,a cyclobutyl group, a cyclopentyl group, a cyclohexyl group, and thelike. Specific examples of the aryl group having 6 to 13 carbon atomsinclude a phenyl group, a naphthyl group, a biphenyl group, a fluorenylgroup, and the like. The above alkyl group, cycloalkyl group, and arylgroup may include one or more substituents, and the substituents may bebonded to each other to form a ring. As the substituent, an alkyl grouphaving 1 to 6 carbon atoms, a cycloalkyl group having 3 to 7 carbonatoms, or an aryl group having 6 to 13 carbon atoms can also beselected. Specific examples of the alkyl group having 1 to 6 carbonatoms include a methyl group, an ethyl group, a propyl group, anisopropyl group, a butyl group, an isobutyl group, a tert-butyl group,an n-hexyl group, and the like. Specific examples of a cycloalkyl grouphaving 3 to 7 carbon atoms include a cyclopropyl group, a cyclobutylgroup, a cyclopentyl group, a cyclohexyl group, and the like. Specificexamples of the aryl group having 6 to 13 carbon atoms include a phenylgroup, a naphthyl group, a biphenyl group, a fluorenyl group, and thelike.

As the arylene group represented by Ar¹ in General Formulae (G0) to(G4), any of groups represented by Structure Formulae (Ar-1) to (Ar-27)can be used, for example. Note that the group that can be used for Ar¹is not limited to these and may include a substituent.

For example, any of groups represented by Structural Formulae (R-1) to(R-29) can be used for the alkyl group, the cycloalkyl group, or thearyl group represented by R¹ to R²⁰ in General Formulae (G1) and (G2),R¹ to R¹⁵ in General Formula (G0), and R¹⁵ represented by GeneralFormulae (G3) and (G4). Note that the group that can be used as thealkyl group, the cycloalkyl group, or the aryl group is not limited tothese and may include a substituent.

<<<Specific Examples of Compounds>>>

Specific examples of structures of the compounds represented by GeneralFormulae (G0) to (G4) include compounds represented by StructuralFormulae (100) to (147). Note that the compounds represented by GeneralFormulae (G0) to (G4) are not limited to the following examples.

Compound Example 2

Note that although the host material 132 preferably has a smalldifference between the singlet excitation energy level and the tripletexcitation energy level, the host material 132 need not necessarily havehigh reverse intersystem crossing efficiency, a high luminescencequantum yield, or a function of exhibiting thermally activated delayedfluorescence. In that case, the host material 132 preferably has astructure in which a skeleton having the π-electron deficientheteroaromatic ring and at least one of a skeleton having the π-electronrich heteroaromatic ring and an aromatic amine skeleton are bonded toeach other through a structure including at least one of a m-phenylenegroup and an o-phenylene group. Alternatively, the skeletons arepreferably bonded to each other through a biphenyldiyl group.Alternatively, the host material 132 preferably has a structure in whichthe skeletons are bonded to each other through an arylene group havingat least one of a m-phenylene group and a o-phenylene group, and morepreferably, the arylene group is a biphenyldiyl group. The host material132 having the above-described structure can have a high T1 level. Notethat also in this case, it is preferable that the skeleton having theπ-electron deficient heteroaromatic ring have at least one of a diazineskeleton (a pyrimidine skeleton, a pyrazine skeleton, or a pyridazineskeleton) and a triazine skeleton. The skeleton having the π-electronrich heteroaromatic ring preferably includes at least one of an acridineskeleton, a phenoxazine skeleton, a phenothiazine skeleton, a furanskeleton, a thiophene skeleton, and a pyrrole skeleton. As the furanskeleton, a dibenzofuran skeleton is preferable. As the thiopheneskeleton, a dibenzothiophene skeleton is preferable. As the pyrroleskeleton, an indole skeleton, a carbazole skeleton, or a9-phenyl-3,3′-bi-9H-carbazole skeleton is particularly preferred. As thearomatic amine skeleton, a tertiary amine, which does not include an NHbond, is preferable, and a triarylamine skeleton is particularlypreferable. As aryl groups of the triarylamine skeleton, substituted orunsubstituted aryl groups having 6 to 13 carbon atoms that form ringsare preferable and examples thereof include phenyl groups, naphthylgroups, and fluorenyl groups.

As examples of the above-described aromatic amine skeleton and theskeleton having the π-electron rich heteroaromatic ring, skeletonsrepresented by General Formulae (401) to (417) are given. Note that X inGeneral Formulae (413) to (416) represents an oxygen atom or a sulfuratom.

As examples of the above-described skeleton having the π-electrondeficient heteroaromatic ring, skeletons represented by General Formulae(201) to (218) are given.

In the case where a skeleton having a hole-transport property (e.g., atleast one of the skeleton having the π-electron rich heteroaromatic ringand the aromatic amine skeleton) and a skeleton having anelectron-transport property (e.g., the skeleton having the π-electrondeficient heteroaromatic ring) are bonded to each other through abonding group including at least one of the m-phenylene group and theo-phenylene group, through a biphenyldiyl group as a bonding group, orthrough a bonding group including an arylene group including at leastone of the m-phenylene group and the o-phenylene group, examples of thebonding group include skeletons represented by General Formulae (301) to(315). Examples of the above-described arylene group include a phenylenegroup, a biphenyldiyl group, a naphthalenediyl group, a fluorenediylgroup, and a phenanthrenediyl group.

The above-described aromatic amine skeleton (e.g., the triarylamineskeleton), π-electron rich heteroaromatic ring skeleton (e.g., a ringincluding at least one of the acridine skeleton, the phenoxazineskeleton, the phenothiazine skeleton, the furan skeleton, the thiopheneskeleton, and the pyrrole skeleton), and π-electron deficientheteroaromatic ring skeleton (e.g., a ring including at least one of thediazine skeleton and the triazine skeleton) or General Formulae (401) to(417), General Formulae (201) to (218), and General Formulae (301) to(315) may each have a substituent. As the substituent, an alkyl grouphaving 1 to 6 carbon atoms, a cycloalkyl group having 3 to 6 carbonatoms, or a substituted or unsubstituted aryl group having 6 to 12carbon atoms can also be selected. Specific examples of the alkyl grouphaving 1 to 6 carbon atoms include a methyl group, an ethyl group, apropyl group, an isopropyl group, a butyl group, an isobutyl group, atert-butyl group, an n-hexyl group, and the like. Specific examples of acycloalkyl group having 3 to 6 carbon atoms include a cyclopropyl group,a cyclobutyl group, a cyclopentyl group, a cyclohexyl group, and thelike. Specific examples of the aryl group having 6 to 12 carbon atomsare a phenyl group, a naphthyl group, a biphenyl group, and the like.The above substituents may be bonded to each other to form a ring. Forexample, in the case where a carbon atom at the 9-position in a fluoreneskeleton has two phenyl groups as substituents, the phenyl groups arebonded to form a spirofluorene skeleton. Note that an unsubstitutedgroup has an advantage in easy synthesis and an inexpensive rawmaterial.

Furthermore, Ar² represents an arylene group having 6 to 13 carbonatoms. The arylene group may include one or more substituents and thesubstituents may be bonded to each other to form a ring. For example, acarbon atom at the 9-position in a fluorenyl group has two phenyl groupsas substituents and the phenyl groups are bonded to form a spirofluoreneskeleton. Specific examples of the arylene group having 6 to 13 carbonatoms are a phenylene group, a naphthylene group, a biphenylene group, afluorenediyl group, and the like. In the case where the arylene grouphas a substituent, as the substituent, an alkyl group having 1 to 6carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, or an arylgroup having 6 to 12 carbon atoms can also be selected. Specificexamples of the alkyl group having 1 to 6 carbon atoms include a methylgroup, an ethyl group, a propyl group, an isopropyl group, a butylgroup, an isobutyl group, a tert-butyl group, an n-hexyl group, and thelike. Specific examples of a cycloalkyl group having 3 to 6 carbon atomsinclude a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, acyclohexyl group, and the like. Specific examples of the aryl grouphaving 6 to 12 carbon atoms are a phenyl group, a naphthyl group, abiphenyl group, and the like.

As the arylene group represented by Ar², for example, groups representedby Structural Formulae (Ar-1) to (Ar-18) can be used. Note that groupsthat can be used for Ar² are not limited to these.

Furthermore, R²¹ and R²² each independently represent any of hydrogen,an alkyl group having 1 to 6 carbon atoms, a cycloalkyl group having 3to 6 carbon atoms, and a substituted or unsubstituted aryl group having6 to 13 carbon atoms. Specific examples of the alkyl group having 1 to 6carbon atoms include a methyl group, an ethyl group, a propyl group, anisopropyl group, a butyl group, an isobutyl group, a tert-butyl group,an n-hexyl group, and the like. Specific examples of a cycloalkyl grouphaving 3 to 6 carbon atoms include a cyclopropyl group, a cyclobutylgroup, a cyclopentyl group, a cyclohexyl group, and the like. Specificexamples of the aryl group having 6 to 13 carbon atoms include a phenylgroup, a naphthyl group, a biphenyl group, a fluorenyl group, and thelike. The above aryl group or phenyl group may include one or moresubstituents, and the substituents may be bonded to each other to form aring. As the substituent, an alkyl group having 1 to 6 carbon atoms, acycloalkyl group having 3 to 6 carbon atoms, or an aryl group having 6to 12 carbon atoms can also be selected. Specific examples of the alkylgroup having 1 to 6 carbon atoms include a methyl group, an ethyl group,a propyl group, an isopropyl group, a butyl group, an isobutyl group, atert-butyl group, an n-hexyl group, and the like. Specific examples of acycloalkyl group having 3 to 6 carbon atoms include a cyclopropyl group,a cyclobutyl group, a cyclopentyl group, a cyclohexyl group, and thelike. Specific examples of the aryl group having 6 to 12 carbon atomsinclude a phenyl group, a naphthyl group, a biphenyl group, and thelike.

For example, groups represented by Structural Formulae (R-1) to (R-29)can be used as the alkyl group or aryl group represented by R² and R²².Note that the group which can be used as an alkyl group or an aryl groupis not limited thereto.

As a substituent that can be included in General formulae (401) to(417), General formulae (201) to (218), General Formulae (301) to (315),Ar², R²¹, and R²², the alkyl group or aryl group represented byStructural Formulae (R-1) to (R-24) can be used, for example. Note thatthe group which can be used as an alkyl group or an aryl group is notlimited thereto.

It is preferable that the host material 132 and the guest material 131(the phosphorescent material) be selected such that the emission peak ofthe host material 132 overlaps with an absorption band, specifically anabsorption band on the longest wavelength side, of a triplet metal toligand charge transfer (MLCT) transition of the guest material 131 (thephosphorescent material). This makes it possible to provide alight-emitting element with drastically improved emission efficiency.Note that in the case where a thermally activated delayed fluorescentmaterial is used instead of the phosphorescent material, it ispreferable that the absorption band on the longest wavelength side be asinglet absorption band.

<<Guest Material 131>>

As the guest material 131 (the phosphorescent material), an iridium-,rhodium-, or platinum-based organometallic complex or metal complex canbe used; in particular, an organoiridium complex such as aniridium-based ortho-metalated complex is preferable. As anortho-metalated ligand, a 4H-triazole ligand, a 1H-triazole ligand, animidazole ligand, a pyridine ligand, a pyrimidine ligand, a pyrazineligand, an isoquinoline ligand, or the like can be given. As the metalcomplex, a platinum complex having a porphyrin ligand or the like can begiven.

It is preferable that the host material 132 and the guest material 131(the phosphorescent material) be selected such that the HOMO level ofthe guest material 131 (the phosphorescent material) is higher than theHOMO level of the host material 132 and the energy difference betweenthe LUMO level and the HOMO level of the guest material 131 (thephosphorescent material) is greater than the energy difference betweenthe LUMO level and the HOMO level of the host material 132. With thisstructure, a light-emitting element with high emission efficiency andlow driving voltage can be obtained.

Examples of the substance that has an emission peak in the green oryellow wavelength range include organometallic iridium complexes havinga pyrimidine skeleton, such astris(4-methyl-6-phenylpyrimidinato)iridium(III) (abbreviation:Ir(mppm)₃), tris(4-t-butyl-6-phenylpyrimidinato)iridium(III)(abbreviation: Ir(tBuppm)₃),(acetylacetonato)bis(6-methyl-4-phenylpyrimidinato)iridium(III)(abbreviation: Ir(mppm)₂(acac)),(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)(abbreviation: Ir(tBuppm)₂(acac)),(acetylacetonato)bis[4-(2-norbornyl)-6-phenylpyrimidinato]iridium(III)(abbreviation: Ir(nbppm)₂(acac)),(acetylacetonato)bis[5-methyl-6-(2-methylphenyl)-4-phenylpyrimidinato]iridium(III)(abbreviation: Ir(mpmppm)₂(acac)),(acetylacetonato)bis{4,6-dimethyl-2-[6-(2,6-dimethylphenyl)-4-pyrimidinyl-xN3]phenyl-κC}iridium(III)(abbreviation: Ir(dmppm-dmp)₂(acac)),(acetylacetonato)bis(4,6-diphenylpyrimidinato)iridium(III)(abbreviation: Ir(dppm)₂(acac)); organometallic iridium complexes havinga pyrazine skeleton, such as(acetylacetonato)bis(3,5-dimethyl-2-phenylpyrazinato)iridium(III)(abbreviation: Ir(mppr-Me)₂(acac)) and(acetylacetonato)bis(5-isopropyl-3-methyl-2-phenylpyrazinato)iridium(III)(abbreviation: Ir(mppr-iPr)(acac)); organometallic iridium complexeshaving a pyridine skeleton, such astris(2-phenylpyridinato-N,C^(2′))iridium(III) (abbreviation: Ir(ppy)₃),bis(2-phenylpyridinato-N,C^(2′))iridium(III) acetylacetonate(abbreviation: Ir(ppy)₂(acac)), bis(benzo[h]quinolinato)iridium(III)acetylacetonate (abbreviation: Ir(bzq)₂(acac)),tris(benzo[h]quinolinato)iridium(III) (abbreviation: Ir(bzq)₃),tris(2-phenylquinolinato-N,C^(2′))iridium(III) (abbreviation: Ir(pq)₃),and bis(2-phenylquinolinato-N,C^(2′))iridium(III) acetylacetonate(abbreviation: Ir(pq)₂(acac)); organometallic iridium complexes such asbis(2,4-diphenyl-1,3-oxazolato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: Ir(dpo)₂(acac)),bis{2-[4′-(perfluorophenyl)phenyl]pyridinato-N,C^(2′)}iridium(III)acetylacetonate(abbreviation: Ir(p-PF-ph)₂(acac)), andbis(2-phenylbenzothiazolato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: Ir(bt)₂(acac)); and a rare earth metal complex such astris(acetylacetonato) (monophenanthroline)terbium(II) (abbreviation:Tb(acac)₃(Phen)). Among the materials given above, the organometalliciridium complex having a pyrimidine skeleton has distinctively highreliability and emission efficiency and is thus especially preferable.

Examples of the substance that has an emission peak in the yellow or redwavelength range include organometallic iridium complexes having apyrimidine skeleton, such as(diisobutyrylmethanato)bis[4,6-bis(3-methylphenyl)pyrimidinato]iridium(III)(abbreviation: Ir(5mdppm)₂(dibm)),bis[4,6-bis(3-methylphenyl)pyrimidinato](dipivaloylmethanato)iridium(III)(abbreviation: Ir(5mdppm)₂(dpm)), andbis[4,6-di(naphthalen-1-yl)pyrimidinato](dipivaloylmethanato)iridium(III) (abbreviation: Ir(d1npm)₂(dpm));organometallic iridium complexes having a pyrazine skeleton, such as(acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III)(abbreviation: Ir(tppr)₂(acac)), bis(2,3,5-triphenylpyrazinato)(dipivaloylmethanato)iridium(III) (abbreviation: Ir(tppr)₂(dpm)), and(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: Ir(Fdpq)₂(acac)); organometallic iridium complexes havinga pyridine skeleton, such astris(1-phenylisoquinolinato-N,C^(2′))iridium(III) (abbreviation:Ir(piq)₃) andbis(1-phenylisoquinolinato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: Ir(piq)₂(acac)); a platinum complex such as2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin platinum(II)(abbreviation: PtOEP); and rare earth metal complexes such astris(1,3-diphenyl-1,3-propanedionato) (monophenanthroline)europium(III)(abbreviation: Eu(DBM)₃(Phen)) andtris[1-(2-thenoyl)-3,3,3-trifluoroacetonato](monophenanthroline)europium(III)(abbreviation: Eu(TTA)₃(Phen)). Among the materials given above, theorganometallic iridium complex having a pyrimidine skeleton hasdistinctively high reliability and emission efficiency and is thusespecially preferable. Further, the organometallic iridium complexeshaving pyrazine skeletons can provide red light emission with favorablechromaticity.

Examples of the substance that has an emission peak in the blue or greenwavelength range include organometallic iridium complexes having a4H-triazole skeleton, such astris{2-[5-(2-methylphenyl)-4-(2,6-dimethylphenyl)-4H-1,2,4-triazol-3-yl-κN2]phenyl-κC}iridium(III)(abbreviation: Ir(mpptz-dmp)₃),tris(5-methyl-3,4-diphenyl-4H-1,2,4-triazolato)iridium(III)(abbreviation: Ir(Mptz)₃),tris[4-(3-biphenyl)-5-isopropyl-3-phenyl-4H-1,2,4-triazolato]iridium(III)(abbreviation: Ir(iPrptz-3b)₃), andtris[3-(5-biphenyl)-5-isopropyl-4-phenyl-4H-1,2,4-triazolato]iridium(III)(abbreviation: Ir(iPr5btz)₃); organometallic iridium complexes having a4H-triazole skeleton with an electron-withdrawing group, such as(OC-6-22)-tris(5-cyano-2-[4-(2,6-diisopropylphenyl)-5-(2-methylphenyl)-4H-1,2,4-triazol-3-yl-κN²]phenyl-κC)iridium(III)(abbreviation: fac-Ir(mpCNptz-diPrp)₃),(OC-6-21)-tris{5-cyano-2-[4-(2,6-diisopropylphenyl)-5-(2-methylphenyl)-4H-1,2,4-triazol-3-yl-κN²]phenyl-κC}iridium(III)(abbreviation: mer-Ir(mpCNptz-diPrp)₃), andtris(2-[4-(4-cyano-2,6-diisobutylphenyl)-5-(2-methylphenyl)-4H-1,2,4-triazol-3-yl-κN²]phenyl-κC)iridium(III)(abbreviation: Ir(mpptz-diBuCNp)₃); organometallic iridium complexeshaving a 1H-triazole skeleton, such astris[3-methyl-1-(2-methylphenyl)-5-phenyl-1H-1,2,4-triazolato]iridium(III)(abbreviation: Ir(Mptz1-mp)₃) andtris(1-methyl-5-phenyl-3-propyl-1H-1,2,4-triazolato)iridium(III)(abbreviation: Ir(Prptz1-Me)₃); organometallic iridium complexes havingan imidazole skeleton, such asfac-tris[1-(2,6-diisopropylphenyl)-2-phenyl-1H-imidazole]iridium(III)(abbreviation: Ir(iPrpmi)₃) andtris[3-(2,6-dimethylphenyl)-7-methylimidazo[1,2-f]phenanthridinato]iridium(III)(abbreviation: Ir(dmpimpt-Me)₃); and organometallic iridium complexes inwhich a phenylpyridine derivative having an electron-withdrawing groupis a ligand, such asbis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)tetrakis(1-pyrazolyl)borate (abbreviation: FIr6),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III) picolinate(abbreviation: FIrpic),bis{2-[3′,5′-bis(trifluoromethyl)phenyl]pyridinato-N,C^(2′)}iridium(III)picolinate(abbreviation: Ir(CF₃ppy)₂(pic)), andbis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)acetylacetonate (abbreviation: FIr(acac)). Among the substances givenabove, the organometallic iridium complexes including anitrogen-containing five-membered heterocyclic skeleton, such as a4H-triazole skeleton, a 1H-triazole skeleton, or an imidazole skeletonhave high triplet excitation energy, reliability, and emissionefficiency and are thus especially preferable.

The above-described organometallic iridium complexes that have anitrogen-containing five-membered heterocyclic skeleton such as a4H-triazole skeleton, a 1H-triazole skeleton, and an imidazole skeletonand the above-described iridium complexes that have a pyridine skeletonhave ligands with a low electron-accepting property and easily have ahigh HOMO level; therefore, those complexes are suitable for oneembodiment of the present invention.

Among the above organometallic iridium complexes that have anitrogen-containing five-membered heterocyclic skeleton, at least theiridium complexes that have a substituent including a cyano group can besuitably used for the light-emitting element of one embodiment of thepresent invention because they have adequately lowered LUMO and HOMOlevels owing to a high electron-withdrawing property of the cyano group.Furthermore, since the iridium complex has a high triplet excitationenergy level, a light-emitting element including the iridium complex canemit blue light with high emission efficiency. Since the iridium complexis highly resistant to repetition of oxidation and reduction, alight-emitting element including the iridium complex can have a longdriving lifetime.

Note that the iridium complex preferably includes a ligand in which anaryl group including a cyano group is bonded to the nitrogen-containingfive-membered heterocyclic skeleton, and the number of carbon atoms ofthe aryl group is preferably 6 to 13 in terms of stability andreliability of the element characteristics. In that case, the iridiumcomplex can be vacuum-evaporated at a relatively low temperature, andaccordingly is unlikely to deteriorate due to pyrolysis or the like atevaporation.

The iridium complex including a ligand in which a cyano group is bondedto a nitrogen atom of a nitrogen-containing five-membered heterocyclicskeleton through an arylene group can keep high triplet excitationenergy level, and thus can be preferably used in a light-emittingelement emitting high-energy light such as blue light. Thelight-emitting element including the iridium complex can emithigh-energy light such as blue light with higher efficiency than alight-emitting element which does not include a cyano group. Moreover,by bonding a cyano group to a particular site as described above, ahighly reliable light-emitting element emitting high-energy light suchas blue light can be obtained. Note that it is preferable that thenitrogen-containing five-membered heterocyclic skeleton and the cyanogroup be bonded through an arylene group such as a phenylene group.

When the number of carbon atoms of the arylene group is 6 to 13, theiridium complex is a compound with a relatively low molecular weight andaccordingly suitable for vacuum evaporation (capable of beingvacuum-evaporated at a relatively low temperature). In general, a lowermolecular weight compound tends to have lower heat resistance after filmformation. However, even with a low molecular weight, the iridiumcomplex has an advantage in that sufficient heat resistance can beensured because the iridium complex includes a plurality of ligands.

That is, the iridium complex has a feature of a high triplet excitationenergy level, in addition to the ease of evaporation and electrochemicalstability. Therefore, it is preferable to use the iridium complex as aguest material in a light-emitting layer in a light-emitting element ofone embodiment of the present invention, particularly in a bluelight-emitting element.

Examples of Iridium Complex

The above-described iridium complex is represented by General Formula(G11).

In General Formula (G11), each of Ar¹¹ and Ar¹² independently representsa substituted or unsubstituted aryl group having 6 to 13 carbon atoms.Specific examples of the aryl group having 6 to 13 carbon atoms includea phenyl group, a naphthyl group, a biphenyl group, and a fluorenylgroup. In the case where the aryl group has a substituent, as thesubstituent, an alkyl group having 1 to 6 carbon atoms, a cycloalkylgroup having 3 to 6 carbon atoms, or a substituted or unsubstituted arylgroup having 6 to 13 carbon atoms can also be selected. Specificexamples of the alkyl group having 1 to 6 carbon atoms include a methylgroup, an ethyl group, a propyl group, an isopropyl group, a butylgroup, an isobutyl group, a tert-butyl group, and an n-hexyl group.Specific examples of a cycloalkyl group having 3 to 6 carbon atomsinclude a cyclopropyl group, a cyclobutyl group, a cyclopentyl group,and a cyclohexyl group. Specific examples of the aryl group having 6 to13 carbon atoms include a phenyl group, a naphthyl group, a biphenylgroup, and a fluorenyl group.

Each of Q¹ and Q² independently represents N or C—R, and R representshydrogen, an alkyl group having 1 to 6 carbon atoms, a haloalkyl grouphaving 1 to 6 carbon atoms, or a substituted or unsubstituted aryl grouphaving 6 to 13 carbon atoms. At least one of Q¹ and Q² includes C—R.Specific examples of the alkyl group having 1 to 6 carbon atoms includea methyl group, an ethyl group, a propyl group, an isopropyl group, abutyl group, an isobutyl group, a tert-butyl group, and an n-hexylgroup. The haloalkyl group having 1 to 6 carbon atoms is an alkyl groupin which at least one hydrogen is replaced with a Group 17 element(fluorine, chlorine, bromine, iodine, or astatine). Examples of thehaloalkyl group having 1 to 6 carbon atoms include an alkyl fluoridegroup, an alkyl chloride group, an alkyl bromide group, and an alkyliodide group. Specific examples thereof include a methyl fluoride group,a methyl chloride group, an ethyl fluoride group, and an ethyl chloridegroup. Note that the number of halogen elements and the kinds thereofmay be one or two or more. Specific examples of the aryl group having 6to 13 carbon atoms include a phenyl group, a naphthyl group, a biphenylgroup, and a fluorenyl group. The aryl group may have a substituent, andsubstituents of the aryl group may be bonded to form a ring. As thesubstituent, an alkyl group having 1 to 6 carbon atoms, a cycloalkylgroup having 3 to 6 carbon atoms, or an aryl group having 6 to 13 carbonatoms can also be selected. Specific examples of the alkyl group having1 to 6 carbon atoms include a methyl group, an ethyl group, a propylgroup, an isopropyl group, a butyl group, an isobutyl group, atert-butyl group, and an n-hexyl group. Specific examples of thecycloalkyl group having 3 to 6 carbon atoms include a cyclopropyl group,a cyclobutyl group, a cyclopentyl group, and a cyclohexyl group.Specific examples of the aryl group having 6 to 13 carbon atoms includea phenyl group, a naphthyl group, a biphenyl group, and a fluorenylgroup.

At least one of the aryl groups represented by Ar¹¹ and Ar¹² and thearyl group represented by R includes a cyano group.

An iridium complex that can be favorably used for a light-emittingelement of one embodiment of the present invention is preferably anortho-metalated complex. This iridium complex is represented by GeneralFormula (G12).

In General Formula (G12), Ar¹¹ represents a substituted or unsubstitutedaryl group having 6 to 13 carbon atoms. Specific examples of the arylgroup having 6 to 13 carbon atoms include a phenyl group, a naphthylgroup, a biphenyl group, and a fluorenyl group. In the case where thearyl group has a substituent, as the substituent, an alkyl group having1 to 6 carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, or asubstituted or unsubstituted aryl group having 6 to 13 carbon atoms canalso be selected. Specific examples of the alkyl group having 1 to 6carbon atoms include a methyl group, an ethyl group, a propyl group, anisopropyl group, a butyl group, an isobutyl group, a tert-butyl group,and an n-hexyl group. Specific examples of a cycloalkyl group having 3to 6 carbon atoms include a cyclopropyl group, a cyclobutyl group, acyclopentyl group, and a cyclohexyl group. Specific examples of the arylgroup having 6 to 13 carbon atoms include a phenyl group, a naphthylgroup, a biphenyl group, and a fluorenyl group.

Each of R³¹ to R³⁴ independently represents any of hydrogen, an alkylgroup having 1 to 6 carbon atoms, a cycloalkyl group having 3 to 6carbon atoms, a substituted or unsubstituted aryl group having 6 to 13carbon atoms, and a cyano group. Specific examples of the alkyl grouphaving 1 to 6 carbon atoms include a methyl group, an ethyl group, apropyl group, an isopropyl group, a butyl group, an isobutyl group, atert-butyl group, and an n-hexyl group. Specific examples of acycloalkyl group having 3 to 6 carbon atoms include a cyclopropyl group,a cyclobutyl group, a cyclopentyl group, and a cyclohexyl group.Specific examples of the aryl group having 6 to 13 carbon atoms includea phenyl group, a naphthyl group, a biphenyl group, and a fluorenylgroup. The case where all of R³¹ to R³⁴ are hydrogen has advantages ineasiness of synthesis and material cost.

Each of Q¹ and Q² independently represents N or C—R, and R representshydrogen, an alkyl group having 1 to 6 carbon atoms, a haloalkyl grouphaving 1 to 6 carbon atoms, or a substituted or unsubstituted aryl grouphaving 6 to 13 carbon atoms. At least one of Q¹ and Q² includes C—R.Specific examples of the alkyl group having 1 to 6 carbon atoms includea methyl group, an ethyl group, a propyl group, an isopropyl group, abutyl group, an isobutyl group, a tert-butyl group, and an n-hexylgroup. The haloalkyl group having 1 to 6 carbon atoms is an alkyl groupin which at least one hydrogen is replaced with a Group 17 element(fluorine, chlorine, bromine, iodine, or astatine). Examples of thehaloalkyl group having 1 to 6 carbon atoms include an alkyl fluoridegroup, an alkyl chloride group, an alkyl bromide group, and an alkyliodide group. Specific examples thereof include a methyl fluoride group,a methyl chloride group, an ethyl fluoride group, and an ethyl chloridegroup. Note that the number of halogen elements and the kinds thereofmay be one or two or more. Specific examples of the aryl group having 6to 13 carbon atoms include a phenyl group, a naphthyl group, a biphenylgroup, and a fluorenyl group. The aryl group may have a substituent, andsubstituents of the aryl group may be bonded to form a ring. As thesubstituent, an alkyl group having 1 to 6 carbon atoms, a cycloalkylgroup having 3 to 6 carbon atoms, or an aryl group having 6 to 13 carbonatoms can also be selected. Specific examples of the alkyl group having1 to 6 carbon atoms include a methyl group, an ethyl group, a propylgroup, an isopropyl group, a butyl group, an isobutyl group, atert-butyl group, and an n-hexyl group. Specific examples of thecycloalkyl group having 3 to 6 carbon atoms include a cyclopropyl group,a cyclobutyl group, a cyclopentyl group, and a cyclohexyl group.Specific examples of the aryl group having 6 to 13 carbon atoms includea phenyl group, a naphthyl group, a biphenyl group, and a fluorenylgroup.

At least one of R³¹ to R³⁴ and the aryl groups represented by Ar¹¹ andR³¹ to R³⁴ and R includes a cyano group.

An iridium complex that can be favorably used for a light-emittingelement of one embodiment of the present invention includes a4H-triazole skeleton as a ligand, which is preferable because theiridium complex can have a high triplet excitation energy level and canbe suitably used in a light-emitting element emitting high-energy lightsuch as blue light. This iridium complex is represented by GeneralFormula (G13).

In General Formula (G13), Ar¹¹ represents a substituted or unsubstitutedaryl group having 6 to 13 carbon atoms. Specific examples of the arylgroup having 6 to 13 carbon atoms include a phenyl group, a naphthylgroup, a biphenyl group, and a fluorenyl group. In the case where thearyl group has a substituent, as the substituent, an alkyl group having1 to 6 carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, or asubstituted or unsubstituted aryl group having 6 to 13 carbon atoms canalso be selected. Specific examples of the alkyl group having 1 to 6carbon atoms include a methyl group, an ethyl group, a propyl group, anisopropyl group, a butyl group, an isobutyl group, a tert-butyl group,and an n-hexyl group. Specific examples of a cycloalkyl group having 3to 6 carbon atoms include a cyclopropyl group, a cyclobutyl group, acyclopentyl group, and a cyclohexyl group. Specific examples of the arylgroup having 6 to 13 carbon atoms include a phenyl group, a naphthylgroup, a biphenyl group, and a fluorenyl group.

Each of R³¹ to R³⁴ independently represents any of hydrogen, an alkylgroup having 1 to 6 carbon atoms, a cycloalkyl group having 3 to 6carbon atoms, a substituted or unsubstituted aryl group having 6 to 13carbon atoms, and a cyano group. Specific examples of the alkyl grouphaving 1 to 6 carbon atoms include a methyl group, an ethyl group, apropyl group, an isopropyl group, a butyl group, an isobutyl group, atert-butyl group, and an n-hexyl group. Specific examples of acycloalkyl group having 3 to 6 carbon atoms include a cyclopropyl group,a cyclobutyl group, a cyclopentyl group, and a cyclohexyl group.Specific examples of the aryl group having 6 to 13 carbon atoms includea phenyl group, a naphthyl group, a biphenyl group, and a fluorenylgroup. The case where all of R³¹ to R³⁴ are hydrogen has advantages ineasiness of synthesis and material cost.

R³⁵ represents any of hydrogen, an alkyl group having 1 to 6 carbonatoms, a haloalkyl group having 1 to 6 carbon atoms, and a substitutedor unsubstituted aryl group having 6 to 13 carbon atoms. Specificexamples of the alkyl group having 1 to 6 carbon atoms include a methylgroup, an ethyl group, a propyl group, an isopropyl group, a butylgroup, an isobutyl group, a tert-butyl group, and an n-hexyl group. Thehaloalkyl group having 1 to 6 carbon atoms is an alkyl group in which atleast one hydrogen is replaced with a Group 17 element (fluorine,chlorine, bromine, iodine, or astatine). Examples of the haloalkyl grouphaving 1 to 6 carbon atoms include an alkyl fluoride group, an alkylchloride group, an alkyl bromide group, and an alkyl iodide group.Specific examples thereof include a methyl fluoride group, a methylchloride group, an ethyl fluoride group, and an ethyl chloride group.Note that the number of halogen elements and the kinds thereof may beone or two or more. Specific examples of the aryl group having 6 to 13carbon atoms include a phenyl group, a naphthyl group, a biphenyl group,and a fluorenyl group. The aryl group may have a substituent, andsubstituents of the aryl group may be bonded to form a ring. As thesubstituent, an alkyl group having 1 to 6 carbon atoms, a cycloalkylgroup having 3 to 6 carbon atoms, or an aryl group having 6 to 13 carbonatoms can also be selected. Specific examples of the alkyl group having1 to 6 carbon atoms include a methyl group, an ethyl group, a propylgroup, an isopropyl group, a butyl group, an isobutyl group, atert-butyl group, and an n-hexyl group. Specific examples of thecycloalkyl group having 3 to 6 carbon atoms include a cyclopropyl group,a cyclobutyl group, a cyclopentyl group, and a cyclohexyl group.Specific examples of the aryl group having 6 to 13 carbon atoms includea phenyl group, a naphthyl group, a biphenyl group, and a fluorenylgroup.

At least one of R³¹ to R³⁴ and the aryl groups represented by Ar¹¹ andR³¹ to R³⁵ includes a cyano group.

An iridium complex that can be favorably used for a light-emittingelement of one embodiment of the present invention includes an imidazoleskeleton as a ligand, which is preferable because the iridium complexcan have a high triplet excitation energy level and can be suitably usedin a light-emitting element emitting high-energy light such as bluelight. This iridium complex is represented by General Formula (G14).

In General Formula (G14), Ar¹¹ represents a substituted or unsubstitutedaryl group having 6 to 13 carbon atoms. Specific examples of the arylgroup having 6 to 13 carbon atoms include a phenyl group, a naphthylgroup, a biphenyl group, and a fluorenyl group. In the case where thearyl group has a substituent, as the substituent, an alkyl group having1 to 6 carbon atoms, a cycloalkyl group having 3 to 6 carbon atoms, or asubstituted or unsubstituted aryl group having 6 to 13 carbon atoms canalso be selected. Specific examples of the alkyl group having 1 to 6carbon atoms include a methyl group, an ethyl group, a propyl group, anisopropyl group, a butyl group, an isobutyl group, a tert-butyl group,and an n-hexyl group. Specific examples of a cycloalkyl group having 3to 6 carbon atoms include a cyclopropyl group, a cyclobutyl group, acyclopentyl group, and a cyclohexyl group. Specific examples of the arylgroup having 6 to 13 carbon atoms include a phenyl group, a naphthylgroup, a biphenyl group, and a fluorenyl group.

Each of R³¹ to R³⁴ independently represents any of hydrogen, an alkylgroup having 1 to 6 carbon atoms, a cycloalkyl group having 3 to 6carbon atoms, and a substituted or unsubstituted aryl group having 6 to13 carbon atoms. Specific examples of the alkyl group having 1 to 6carbon atoms include a methyl group, an ethyl group, a propyl group, anisopropyl group, a butyl group, an isobutyl group, a tert-butyl group,and an n-hexyl group. Specific examples of a cycloalkyl group having 3to 6 carbon atoms include a cyclopropyl group, a cyclobutyl group, acyclopentyl group, and a cyclohexyl group. Specific examples of the arylgroup having 6 to 13 carbon atoms include a phenyl group, a naphthylgroup, a biphenyl group, and a fluorenyl group. The case where all ofR³¹ to R³⁴ are hydrogen has advantages in easiness of synthesis andmaterial cost.

Each of R³⁵ and R³⁶ independently represents any of hydrogen, an alkylgroup having 1 to 6 carbon atoms, a haloalkyl group having 1 to 6 carbonatoms, and a substituted or unsubstituted aryl group having 6 to 13carbon atoms. Specific examples of the alkyl group having 1 to 6 carbonatoms include a methyl group, an ethyl group, a propyl group, anisopropyl group, a butyl group, an isobutyl group, a tert-butyl group,and an n-hexyl group. The haloalkyl group having 1 to 6 carbon atoms isan alkyl group in which at least one hydrogen is replaced with a Group17 element (fluorine, chlorine, bromine, iodine, or astatine). Examplesof the haloalkyl group having 1 to 6 carbon atoms include an alkylfluoride group, an alkyl chloride group, an alkyl bromide group, and analkyl iodide group. Specific examples thereof include a methyl fluoridegroup, a methyl chloride group, an ethyl fluoride group, and an ethylchloride group. Note that the number of halogen elements and the kindsthereof may be one or two or more. Specific examples of the aryl grouphaving 6 to 13 carbon atoms include a phenyl group, a naphthyl group, abiphenyl group, and a fluorenyl group. The aryl group may have asubstituent, and substituents of the aryl group may be bonded to form aring. As the substituent, an alkyl group having 1 to 6 carbon atoms, acycloalkyl group having 3 to 6 carbon atoms, or an aryl group having 6to 13 carbon atoms can also be selected. Specific examples of the alkylgroup having 1 to 6 carbon atoms include a methyl group, an ethyl group,a propyl group, an isopropyl group, a butyl group, an isobutyl group, atert-butyl group, and an n-hexyl group. Specific examples of thecycloalkyl group having 3 to 6 carbon atoms include a cyclopropyl group,a cyclobutyl group, a cyclopentyl group, and a cyclohexyl group.Specific examples of the aryl group having 6 to 13 carbon atoms includea phenyl group, a naphthyl group, a biphenyl group, and a fluorenylgroup.

At least one of R³¹ to R³⁴ and the aryl groups represented by Ar¹¹ andR³¹ to R³⁶ includes a cyano group.

An iridium complex that can be favorably used for a light-emittingelement of one embodiment of the present invention includes anitrogen-containing five-membered heterocyclic skeleton, and an arylgroup bonded to nitrogen of the skeleton is preferably a substituted orunsubstituted phenyl group. In that case, the iridium complex can bevacuum-evaporated at a relatively low temperature and can have a hightriplet excitation energy level, and accordingly can be used in alight-emitting element emitting high-energy light such as blue light.The iridium complex is represented by General Formula (G15) or (G16).

In General Formula (G15), each of R³⁷ and R⁴¹ represents an alkyl grouphaving 1 to 6 carbon atoms, and R³⁷ and R⁴¹ have the same structure.Specific examples of the alkyl group having 1 to 6 carbon atoms includea methyl group, an ethyl group, a propyl group, an isopropyl group, abutyl group, an isobutyl group, a tert-butyl group, and an n-hexylgroup.

Each of R³⁸ to R⁴⁰ independently represents hydrogen, an alkyl grouphaving 1 to 6 carbon atoms, a cycloalkyl group having 3 to 6 carbonatoms, a substituted or unsubstituted phenyl group, or a cyano group.Specific examples of the alkyl group having 1 to 6 carbon atoms includea methyl group, an ethyl group, a propyl group, an isopropyl group, abutyl group, an isobutyl group, a tert-butyl group, and an n-hexylgroup. Specific examples of a cycloalkyl group having 3 to 6 carbonatoms include a cyclopropyl group, a cyclobutyl group, a cyclopentylgroup, and a cyclohexyl group. Note that at least one of R³¹ to R⁴⁰includes a cyano group.

Each of R³¹ to R³⁴ independently represents any of hydrogen, an alkylgroup having 1 to 6 carbon atoms, a cycloalkyl group having 3 to 6carbon atoms, and a substituted or unsubstituted aryl group having 6 to13 carbon atoms. Specific examples of the alkyl group having 1 to 6carbon atoms include a methyl group, an ethyl group, a propyl group, anisopropyl group, a butyl group, an isobutyl group, a tert-butyl group,an n-hexyl group, and the like. Specific examples of a cycloalkyl grouphaving 3 to 6 carbon atoms include a cyclopropyl group, a cyclobutylgroup, a cyclopentyl group, and a cyclohexyl group. Specific examples ofthe aryl group having 6 to 13 carbon atoms include a phenyl group, anaphthyl group, a biphenyl group, a fluorenyl group, and the like. Thecase where all of R³¹ to R³⁴ are hydrogen has advantages in easiness ofsynthesis and material cost.

R³⁵ represents any of hydrogen, an alkyl group having 1 to 6 carbonatoms, a haloalkyl group having 1 to 6 carbon atoms, and a substitutedor unsubstituted aryl group having 6 to 13 carbon atoms. Specificexamples of the alkyl group having 1 to 6 carbon atoms include a methylgroup, an ethyl group, a propyl group, an isopropyl group, a butylgroup, an isobutyl group, a tert-butyl group, an n-hexyl group, and thelike. The haloalkyl group having 1 to 6 carbon atoms is an alkyl groupin which at least one hydrogen is replaced with a Group 17 element(fluorine, chlorine, bromine, iodine, or astatine). Examples of thehaloalkyl group having 1 to 6 carbon atoms include an alkyl fluoridegroup, an alkyl chloride group, an alkyl bromide group, and an alkyliodide group. Specific examples thereof include a methyl fluoride group,a methyl chloride group, an ethyl fluoride group, and an ethyl chloridegroup. Note that the number of halogen elements and the kinds thereofmay be one or two or more. Specific examples of the aryl group having 6to 13 carbon atoms include a phenyl group, a naphthyl group, a biphenylgroup, a fluorenyl group, and the like. The aryl group may have asubstituent, and substituents of the aryl group may be bonded to form aring. As the substituent, an alkyl group having 1 to 6 carbon atoms, acycloalkyl group having 3 to 6 carbon atoms, or an aryl group having 6to 13 carbon atoms can also be selected. Specific examples of the alkylgroup having 1 to 6 carbon atoms include a methyl group, an ethyl group,a propyl group, an isopropyl group, a butyl group, an isobutyl group, atert-butyl group, an n-hexyl group, and the like. Specific examples ofthe cycloalkyl group having 3 to 6 carbon atoms include a cyclopropylgroup, a cyclobutyl group, a cyclopentyl group, and a cyclohexyl group.Specific examples of the aryl group having 6 to 13 carbon atoms includea phenyl group, a naphthyl group, a biphenyl group, a fluorenyl group,and the like.

In General Formula (G16), each of R³⁷ and R⁴ represents an alkyl grouphaving 1 to 6 carbon atoms, and R³⁷ and R⁴¹ have the same structure.Specific examples of the alkyl group having 1 to 6 carbon atoms includea methyl group, an ethyl group, a propyl group, an isopropyl group, abutyl group, an isobutyl group, a tert-butyl group, an n-hexyl group,and the like.

Each of R³¹ to R⁴⁰ independently represents hydrogen, an alkyl grouphaving 1 to 6 carbon atoms, a cycloalkyl group having 3 to 6 carbonatoms, a substituted or unsubstituted phenyl group, or a cyano group.Specific examples of the alkyl group having 1 to 6 carbon atoms includea methyl group, an ethyl group, a propyl group, an isopropyl group, abutyl group, an isobutyl group, a tert-butyl group, an n-hexyl group,and the like. Specific examples of a cycloalkyl group having 3 to 6carbon atoms include a cyclopropyl group, a cyclobutyl group, acyclopentyl group, and a cyclohexyl group. Note that at least one of R³⁸to R⁴⁰ preferably includes a cyano group.

Each of R³¹ to R³⁴ independently represents any of hydrogen, an alkylgroup having 1 to 6 carbon atoms, a cycloalkyl group having 3 to 6carbon atoms, and a substituted or unsubstituted aryl group having 6 to13 carbon atoms. Specific examples of the alkyl group having 1 to 6carbon atoms include a methyl group, an ethyl group, a propyl group, anisopropyl group, a butyl group, an isobutyl group, a tert-butyl group,an n-hexyl group, and the like. Specific examples of a cycloalkyl grouphaving 3 to 6 carbon atoms include a cyclopropyl group, a cyclobutylgroup, a cyclopentyl group, and a cyclohexyl group. Specific examples ofthe aryl group having 6 to 13 carbon atoms include a phenyl group, anaphthyl group, a biphenyl group, a fluorenyl group, and the like. Thecase where all of R³¹ to R³⁴ are hydrogen has advantages in easiness ofsynthesis and material cost.

Each of R³⁵ and R³⁶ independently represents any of hydrogen, an alkylgroup having 1 to 6 carbon atoms, a haloalkyl group having 1 to 6 carbonatoms, and a substituted or unsubstituted aryl group having 6 to 13carbon atoms. Specific examples of the alkyl group having 1 to 6 carbonatoms include a methyl group, an ethyl group, a propyl group, anisopropyl group, a butyl group, an isobutyl group, a tert-butyl group,an n-hexyl group, and the like. The haloalkyl group having 1 to 6 carbonatoms is an alkyl group in which at least one hydrogen is replaced witha Group 17 element (fluorine, chlorine, bromine, iodine, or astatine).Examples of the haloalkyl group having 1 to 6 carbon atoms include analkyl fluoride group, an alkyl chloride group, an alkyl bromide group,and an alkyl iodide group. Specific examples thereof include a methylfluoride group, a methyl chloride group, an ethyl fluoride group, and anethyl chloride group. Note that the number of halogen elements and thekinds thereof may be one or two or more. Specific examples of the arylgroup having 6 to 13 carbon atoms include a phenyl group, a naphthylgroup, a biphenyl group, a fluorenyl group, and the like. The aryl groupmay have a substituent, and substituents of the aryl group may be bondedto form a ring. As the substituent, an alkyl group having 1 to 6 carbonatoms, a cycloalkyl group having 3 to 6 carbon atoms, or an aryl grouphaving 6 to 13 carbon atoms can also be selected. Specific examples ofthe alkyl group having 1 to 6 carbon atoms include a methyl group, anethyl group, a propyl group, an isopropyl group, a butyl group, anisobutyl group, a tert-butyl group, an n-hexyl group, and the like.Specific examples of the cycloalkyl group having 3 to 6 carbon atomsinclude a cyclopropyl group, a cyclobutyl group, a cyclopentyl group,and a cyclohexyl group. Specific examples of the aryl group having 6 to13 carbon atoms include a phenyl group, a naphthyl group, a biphenylgroup, a fluorenyl group, and the like.

Iridium complexes that can be favorably used for light-emitting elementsof one embodiment of the present invention each include a 1H-triazoleskeleton as a ligand, which is preferable because the iridium complexescan have a high triplet excitation energy level and can be suitably usedin light-emitting elements emitting high-energy light such as bluelight. The iridium complexes are represented by General Formula (G17)and (G18).

In General Formula (G17), Ar¹¹ represents a substituted or unsubstitutedaryl group having 6 to 13 carbon atoms. Specific examples of the arylgroup having 6 to 13 carbon atoms include a phenyl group, a naphthylgroup, a biphenyl group, a fluorenyl group, and the like. In the casewhere the aryl group has a substituent, as the substituent, an alkylgroup having 1 to 6 carbon atoms, a cycloalkyl group having 3 to 6carbon atoms, or a substituted or unsubstituted aryl group having 6 to13 carbon atoms can also be selected. Specific examples of the alkylgroup having 1 to 6 carbon atoms include a methyl group, an ethyl group,a propyl group, an isopropyl group, a butyl group, an isobutyl group, atert-butyl group, an n-hexyl group, and the like. Specific examples of acycloalkyl group having 3 to 6 carbon atoms include a cyclopropyl group,a cyclobutyl group, a cyclopentyl group, and a cyclohexyl group.Specific examples of the aryl group having 6 to 13 carbon atoms includea phenyl group, a naphthyl group, a biphenyl group, a fluorenyl group,and the like.

Each of R³¹ to R³⁴ independently represents any of hydrogen, an alkylgroup having 1 to 6 carbon atoms, a cycloalkyl group having 3 to 6carbon atoms, and a substituted or unsubstituted aryl group having 6 to13 carbon atoms. Specific examples of the alkyl group having 1 to 6carbon atoms include a methyl group, an ethyl group, a propyl group, anisopropyl group, a butyl group, an isobutyl group, a tert-butyl group,and an n-hexyl group. Specific examples of a cycloalkyl group having 3to 6 carbon atoms include a cyclopropyl group, a cyclobutyl group, acyclopentyl group, and a cyclohexyl group. Specific examples of the arylgroup having 6 to 13 carbon atoms include a phenyl group, a naphthylgroup, a biphenyl group, and a fluorenyl group. The case where all ofR³¹ to R³⁴ are hydrogen has advantages in easiness of synthesis andmaterial cost.

R³⁶ represents any of hydrogen, an alkyl group having 1 to 6 carbonatoms, a haloalkyl group having 1 to 6 carbon atoms, and a substitutedor unsubstituted aryl group having 6 to 13 carbon atoms. Specificexamples of the alkyl group having 1 to 6 carbon atoms include a methylgroup, an ethyl group, a propyl group, an isopropyl group, a butylgroup, an isobutyl group, a tert-butyl group, and an n-hexyl group. Thehaloalkyl group having 1 to 6 carbon atoms is an alkyl group in which atleast one hydrogen is replaced with a Group 17 element (fluorine,chlorine, bromine, iodine, or astatine). Examples of the haloalkyl grouphaving 1 to 6 carbon atoms include an alkyl fluoride group, an alkylchloride group, an alkyl bromide group, and an alkyl iodide group.Specific examples thereof include a methyl fluoride group, a methylchloride group, an ethyl fluoride group, and an ethyl chloride group.Note that the number of halogen elements and the kinds thereof may beone or two or more. Specific examples of the aryl group having 6 to 13carbon atoms include a phenyl group, a naphthyl group, a biphenyl group,and a fluorenyl group. The aryl group may have a substituent, andsubstituents of the aryl group may be bonded to form a ring. As thesubstituent, an alkyl group having 1 to 6 carbon atoms, a cycloalkylgroup having 3 to 6 carbon atoms, or an aryl group having 6 to 13 carbonatoms can also be selected. Specific examples of the alkyl group having1 to 6 carbon atoms include a methyl group, an ethyl group, a propylgroup, an isopropyl group, a butyl group, an isobutyl group, atert-butyl group, and an n-hexyl group. Specific examples of thecycloalkyl group having 3 to 6 carbon atoms include a cyclopropyl group,a cyclobutyl group, a cyclopentyl group, and a cyclohexyl group.Specific examples of the aryl group having 6 to 13 carbon atoms includea phenyl group, a naphthyl group, a biphenyl group, and a fluorenylgroup.

At least one of R³¹ to R³⁴ and the aryl groups represented by Ar¹¹, R³¹to R³⁴, and R³⁶ includes a cyano group.

In General Formula (G18), each of R³⁷ and R⁴¹ represents an alkyl grouphaving 1 to 6 carbon atoms, and R³⁷ and R⁴¹ have the same structure.Specific examples of the alkyl group having 1 to 6 carbon atoms includea methyl group, an ethyl group, a propyl group, an isopropyl group, abutyl group, an isobutyl group, a tert-butyl group, and an n-hexylgroup.

Each of R³⁸ to R⁴⁰ independently represents hydrogen, an alkyl grouphaving 1 to 6 carbon atoms, a cycloalkyl group having 3 to 6 carbonatoms, a substituted or unsubstituted phenyl group, or a cyano group.Specific examples of the alkyl group having 1 to 6 carbon atoms includea methyl group, an ethyl group, a propyl group, an isopropyl group, abutyl group, an isobutyl group, a tert-butyl group, and an n-hexylgroup. Specific examples of a cycloalkyl group having 3 to 6 carbonatoms include a cyclopropyl group, a cyclobutyl group, a cyclopentylgroup, and a cyclohexyl group. Note that at least one of R³⁸ to R⁴⁰includes a cyano group.

Each of R³¹ to R³⁴ independently represents any of hydrogen, an alkylgroup having 1 to 6 carbon atoms, a cycloalkyl group having 3 to 6carbon atoms, and a substituted or unsubstituted aryl group having 6 to13 carbon atoms. Specific examples of the alkyl group having 1 to 6carbon atoms include a methyl group, an ethyl group, a propyl group, anisopropyl group, a butyl group, an isobutyl group, a tert-butyl group,and an n-hexyl group. Specific examples of a cycloalkyl group having 3to 6 carbon atoms include a cyclopropyl group, a cyclobutyl group, acyclopentyl group, and a cyclohexyl group. Specific examples of the arylgroup having 6 to 13 carbon atoms include a phenyl group, a naphthylgroup, a biphenyl group, and a fluorenyl group. The case where all ofR³¹ to R³⁴ are hydrogen has advantages in easiness of synthesis andmaterial cost.

R³⁶ represents any of hydrogen, an alkyl group having 1 to 6 carbonatoms, a haloalkyl group having 1 to 6 carbon atoms, and a substitutedor unsubstituted aryl group having 6 to 13 carbon atoms. Specificexamples of the alkyl group having 1 to 6 carbon atoms include a methylgroup, an ethyl group, a propyl group, an isopropyl group, a butylgroup, an isobutyl group, a tert-butyl group, and an n-hexyl group. Thehaloalkyl group having 1 to 6 carbon atoms is an alkyl group in which atleast one hydrogen is replaced with a Group 17 element (fluorine,chlorine, bromine, iodine, or astatine). Examples of the haloalkyl grouphaving 1 to 6 carbon atoms include an alkyl fluoride group, an alkylchloride group, an alkyl bromide group, and an alkyl iodide group.Specific examples thereof include a methyl fluoride group, a methylchloride group, an ethyl fluoride group, and an ethyl chloride group.Note that the number of halogen elements and the kinds thereof may beone or two or more. Specific examples of the aryl group having 6 to 13carbon atoms include a phenyl group, a naphthyl group, a biphenyl group,and a fluorenyl group. The aryl group may have a substituent, andsubstituents of the aryl group may be bonded to form a ring. As thesubstituent, an alkyl group having 1 to 6 carbon atoms, a cycloalkylgroup having 3 to 6 carbon atoms, or an aryl group having 6 to 13 carbonatoms can also be selected. Specific examples of the alkyl group having1 to 6 carbon atoms include a methyl group, an ethyl group, a propylgroup, an isopropyl group, a butyl group, an isobutyl group, atert-butyl group, and an n-hexyl group. Specific examples of thecycloalkyl group having 3 to 6 carbon atoms include a cyclopropyl group,a cyclobutyl group, a cyclopentyl group, and a cyclohexyl group.Specific examples of the aryl group having 6 to 13 carbon atoms includea phenyl group, a naphthyl group, a biphenyl group, and a fluorenylgroup.

As an alkyl group and an aryl group represented by R³¹ to R³⁴ in GeneralFormulae (G12) to (G18), for example, groups represented by StructuralFormulae (R-1) to (R-29) can be used. Note that groups that can be usedas the alkyl group and the aryl group are not limited thereto.

For example, groups represented by Structural Formulae (R-12) to (R-29)can be used as an aryl group represented by Ar¹¹ in General Formulae(G11) to (G14) and (G17) and an aryl group represented by Ar¹² inGeneral Formula (G11). Note that groups that can be used as Ar¹¹ andAr¹² are not limited to these groups.

For example, the groups represented by Structural Formulae (R-1) to(R-10) can be used as alkyl groups represented by R³⁷ and R⁴¹ in GeneralFormulae (G15), (G16), and (G18). Note that groups that can be used asthe alkyl group are not limited to these groups.

As the alkyl group or substituted or unsubstituted phenyl grouprepresented by R³⁸ to R⁴⁰ in General Formulae (G15), (G16), and (G18),groups represented by Structure Formulae (R-1) to (R-22) above can beused, for example. Note that groups which can be used as the alkyl groupor the phenyl group are not limited thereto.

For example, groups represented by Structural Formulae (R-1) to (R-29)and Structural Formulae (R-30) to (R-37) can be used as an alkyl group,an aryl group, and a haloalkyl group represented by R³⁵ in GeneralFormulae (G13) to (G16) and R³⁶ in General Formulae (G14) and (G16) to(G18). Note that a group that can be used as the alkyl group, the arylgroup, or the haloalkyl group is not limited to these groups

Specific Examples of Iridium Complexes

Specific examples of structures of the iridium complexes represented byGeneral Formulae (G11) to (G18) are compounds represented by StructuralFormulae (500) to (534). Note that the iridium complexes represented byGeneral Formulae (G11) to (G18) are not limited the examples shownbelow.

The iridium complex described above as an example has relatively lowHOMO and LUMO levels as described above, and is accordingly preferred asa guest material of a light-emitting element of one embodiment of thepresent invention. In that case, the light-emitting element can havehigh emission efficiency. In addition, the iridium complex describedabove as an example has a high triplet excitation energy level, and isaccordingly preferred particularly as a guest material of a bluelight-emitting element. In that case, the blue light-emitting elementcan have high emission efficiency. Moreover, since the iridium complexdescribed above as an example is highly resistant to repetition ofoxidation and reduction, a light-emitting element including the iridiumcomplex can have a long driving lifetime. Therefore, the iridium complexof one embodiment of the present invention is a material suitably usedin a light-emitting element.

As the light-emitting material included in the light-emitting layer 130and the light-emitting layer 135, any material can be used as long asthe material can convert the triplet excitation energy into lightemission. As an example of the material that can convert the tripletexcitation energy into light emission, a thermally activated delayedfluorescent material can be given in addition to the phosphorescentmaterial. Therefore, the term “phosphorescent material” in thedescription can be replaced with the term “thermally activated delayedfluorescent material”.

<<Host Material 133>>

It is preferable that the host material 133, the host material 132, andthe guest material 131 be selected such that the LUMO level of the hostmaterial 133 is higher than the LUMO level of the host material 132 andthe HOMO level of the host material 133 is lower than the HOMO level ofthe guest material 131. With this structure, a light-emitting elementwith high emission efficiency and low driving voltage can be obtained.Note that the material described as an example of the host material 132may be used as the host material 133.

A material having a property of transporting more electrons than holescan be used as the host material 133, and a material having an electronmobility of 1×10⁻⁶ cm²/Vs or higher is preferable. A compound includinga π-electron deficient heteroaromatic ring skeleton such as anitrogen-containing heteroaromatic compound, or a zinc- oraluminum-based metal complex can be used, for example, as the materialwhich easily accepts electrons (the material having anelectron-transport property). Specific examples include a metal complexhaving a quinoline ligand, a benzoquinoline ligand, an oxazole ligand,or a thiazole ligand, an oxadiazole derivative, a triazole derivative, abenzimidazole derivative, a quinoxaline derivative, a dibenzoquinoxalinederivative, a phenanthroline derivative, a pyridine derivative, abipyridine derivative, a pyrimidine derivative, and a triazinederivative.

Specific examples include metal complexes having a quinoline orbenzoquinoline skeleton, such as tris(8-quinolinolato)aluminum(III)(abbreviation: Alq), tris(4-methyl-8-quinolinolato)aluminum(III)(abbreviation: Almq₃), bis(10-hydroxybenzo[h]quinolinato)beryllium(II)(abbreviation: BeBq₂), bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III) (abbreviation: BAlq) andbis(8-quinolinolato)zinc(II) (abbreviation: Znq), and the like.Alternatively, a metal complex having an oxazole-based or thiazole-basedligand, such as bis[2-(2-benzoxazolyl)phenolate]zinc(II) (abbreviation:ZnPBO) or bis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation:ZnBTZ) can be used. Other than such metal complexes, any of thefollowing can be used: heterocyclic compounds such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole (abbreviation:CO11), 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ),9-[4-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)phenyl]-9H-carbazole(abbreviation: CzTAZ1),2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI),2-[3-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole(abbreviation: mDBTBIm-II), bathophenanthroline (abbreviation: BPhen),and bathocuproine (abbreviation: BCP); heterocyclic compounds having adiazine skeleton such as2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mDBTPDBq-II),2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II),2-[3′-(9H-carbazol-9-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mCzBPDBq),2-[4-(3,6-diphenyl-9H-carbazol-9-yl)phenyl]dibenzo[f,h]quinoxaline(abbreviation: 2CzPDBq-III),7-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:7mDBTPDBq-II), 6-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline(abbreviation: 6mDBTPDBq-II),2-[3-(3,9′-bi-9H-carbazol-9-yl)phenyl]dibenzo[f,h]quinoxaline(abbreviation: 2mCzCzPDBq),4,6-bis[3-(phenanthren-9-yl)phenyl]pyrimidine (abbreviation:4,6mPnP2Pm), 4,6-bis[3-(4-dibenzothienyl)phenyl]pyrimidine(abbreviation: 4,6mDBTP2Pm-II), and4,6-bis[3-(9H-carbazol-9-yl)phenyl]pyrimidine (abbreviation:4,6mCzP2Pm); heterocyclic compounds having a triazine skeleton such as2-{4-[3-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}-4,6-diphenyl-1,3,5-triazine(abbreviation: PCCzPTzn); heterocyclic compounds having a pyridineskeleton such as 3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine(abbreviation: 35DCzPPy); and heteroaromatic compounds such as4,4′-bis(5-methylbenzoxazol-2-yl)stilbene (abbreviation: BzOs). Amongthe heterocyclic compounds, the heterocyclic compounds having at leastone of a triazine skeleton, a diazine skeleton (pyrimidine, pyrazine,pyridazine), and a pyridine skeleton are highly reliable and stable andis thus preferably used. In addition, the heterocyclic compounds havingthe skeletons have a high electron-transport property to contribute to areduction in driving voltage. Further alternatively, a high molecularcompound such as poly(2,5-pyridinediyl) (abbreviation: PPy),poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)](abbreviation: PF-Py), orpoly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2′-bipyridine-6,6′-diyl)](abbreviation:PF-BPy) can be used. The substances described here are mainly substanceshaving an electron mobility of 1×10⁻⁶ cm²/Vs or higher. Note that othersubstances may also be used as long as their electron-transportproperties are higher than their hole-transport properties.

As the host material 133, materials having a hole-transport propertygiven below can be used.

A material having a property of transporting more holes than electronscan be used as the hole-transport material, and a material having a holemobility of 1×10⁻⁶ cm²/Vs or higher is preferable. Specifically, anaromatic amine, a carbazole derivative, an aromatic hydrocarbon, astilbene derivative, or the like can be used. Furthermore, thehole-transport material may be a high molecular compound.

Examples of the material having a high hole-transport property areN,N′-di(p-tolyl)-N,N′-diphenyl-p-phenylenediamine (abbreviation:DTDPPA), 4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl(abbreviation: DPAB),N,N-bis{4-[bis(3-methylphenyl)amino]phenyl}-N,N-diphenyl-(1,1′-biphenyl)-4,4′-diamine(abbreviation: DNTPD),1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B), and the like.

Specific examples of the carbazole derivative are3-[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzDPA1),3,6-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzDPA2),3,6-bis[N-(4-diphenylaminophenyl)-N-(1-naphthyl)amino]-9-phenylcarbazole(abbreviation: PCzTPN2),3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation:PCzPCA2),3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1), and the like.

Other examples of the carbazole derivative are4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB),9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA),1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene,andthelike.

Examples of the aromatic hydrocarbons include2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA),2-tert-butyl-9,10-di(1-naphthyl)anthracene,9,10-bis(3,5-diphenylphenyl)anthracene(abbreviation:DPPA),2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene(abbreviation:t-BuDBA), 9,10-di(2-naphthyl)anthracene (abbreviation:DNA), 9,10-diphenylanthracene (abbreviation: DPAnth),2-tert-butylanthracene (abbreviation: t-BuAnth),9,10-bis(4-methyl-1-naphthyl)anthracene (abbreviation: DMNA),2-tert-butyl-9,10-bis[2-(1-naphthyl)phenyl]anthracene,9,10-bis[2-(1-naphthyl)phenyl]anthracene,2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene,2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene,9,9′-bianthryl,10,10′-diphenyl-9,9′-bianthryl,10,10′-bis(2-phenylphenyl)-9,9′-bianthryl,10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracene,tetracene, rubrene, perylene, 2,5,8,11-tetra(tert-butyl)perylene, andthe like. Besides, pentacene, coronene, or the like can also be used.The aromatic hydrocarbon having a hole mobility of 1×10⁻⁶ cm²/Vs or moreand having 14 to 42 carbon atoms is particularly preferable.

The aromatic hydrocarbon may have a vinyl skeleton. As aromatichydrocarbon having a vinyl group, the following is given, for example:4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation: DPVBi);9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene (abbreviation: DPVPA);and the like.

Moreover, a high molecular compound such as poly(N-vinylcarbazole)(abbreviation: PVK), poly(4-vinyltriphenylamine) (abbreviation: PVTPA),poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide](abbreviation:PTPDMA), or poly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine(abbreviation: Poly-TPD) can also be used.

Examples of the material having a high hole-transport property includearomatic amine compounds such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB orα-NPD),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(carbazol-9-yl)triphenylamine(abbreviation: TCTA),4,4′,4″-tris[N-(1-naphthyl)-N-phenylamino]triphenylamine (abbreviation:1′-TNATA), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine (abbreviation:TDATA), 4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA),4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP), 4-phenyl-3′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: mBPAFLP),N-(9,9-dimethyl-9H-fluoren-2-yl)-N-{9,9-dimethyl-2-[N′-phenyl-N′-(9,9-dimethyl-9H-fluoren-2-yl)amino]-9H-fluoren-7-yl}phenylamine(abbreviation: DFLADFL),N-(9,9-dimethyl-2-diphenylamino-9H-fluoren-7-yl)diphenylamine(abbreviation: DPNF),2-[N-(4-diphenylaminophenyl)-N-phenylamino]spiro-9,9′-bifluorene(abbreviation: DPASF),4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation:PCBA1BP), 4,4′-diphenyl-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBBi1BP),4-(1-naphthyl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBANB),4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBNBB),4-phenyldiphenyl-(9-phenyl-9H-carbazol-3-yl)amine (abbreviation:PCA1BP),N,N′-bis(9-phenylcarbazol-3-yl)-N,N′-diphenylbenzene-1,3-diamine(abbreviation: PCA2B),N,N,N″-triphenyl-N,N,N′″-tris(9-phenylcarbazol-3-yl)benzene-1,3,5-triamine(abbreviation: PCA3B),N-(4-biphenyl)-N-(9,9-dimethyl-9H-fluoren-2-yl)-9-phenyl-9H-carbazol-3-amine(abbreviation: PCBiF),N-(1,1′-biphenyl-4-yl)-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]-9,9-dimethyl-9H-fluoren-2-amine(abbreviation: PCBBiF),9,9-dimethyl-N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]fluoren-2-amine(abbreviation: PCBAF),N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]spiro-9,9′-bifluoren-2-amine(abbreviation: PCBASF),2-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]spiro-9,9′-bifluorene(abbreviation: PCASF),2,7-bis[N-(4-diphenylaminophenyl)-N-phenylamino]spiro-9,9′-bifluorene(abbreviation: DPA2SF),N-[4-(9H-carbazol-9-yl)phenyl]-N-(4-phenyl)phenylaniline (abbreviation:YGABP), andN,N′-bis[4-(carbazol-9-yl)phenyl]-N,N-diphenyl-9,9-dimethylfluorene-2,7-diamine(abbreviation: YGA2F). Other examples are amine compounds, carbazolecompounds, thiophene compounds, furan compounds, fluorene compounds;triphenylene compounds; phenanthrene compounds, and the like such as3-[4-(1-naphthyl)-phenyl]-9-phenyl-9H-carbazole (abbreviation: PCPN),3-[4-(9-phenanthryl)-phenyl]-9-phenyl-9H-carbazole (abbreviation:PCPPn), 3,3′-bis(9-phenyl-9H-carbazole) (abbreviation: PCCP),1,3-bis(N-carbazolyl)benzene (abbreviation: mCP),3,6-bis(3,5-diphenylphenyl)-9-phenylcarbazole (abbreviation: CzTP),3,6-di(9H-carbazol-9-yl)-9-phenyl-9H-carbazole (abbreviation: PhCzGI),2,8-di(9H-carbazol-9-yl)-dibenzothiophene (abbreviation: Cz2DBT),4-{3-[3-(9-phenyl-9H-fluoren-9-yl)phenyl]phenyl}dibenzofuran(abbreviation: mmDBFFLBi-II),4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzofuran) (abbreviation: DBF3P-II),1,3,5-tri(dibenzothiophen-4-yl)benzene (abbreviation: DBT3P-II),2,8-diphenyl-4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]dibenzothiophene(abbreviation: DBTFLP-III),4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-6-phenyldibenzothiophene(abbreviation: DBTFLP-IV), and4-[3-(triphenylene-2-yl)phenyl]dibenzothiophene (abbreviation:mDBTPTp-II). Among the above compounds, compounds including at least oneof a pyrrole skeleton, a furan skeleton, a thiophene skeleton, and anaromatic amine skeleton are preferred because of their high stabilityand reliability. In addition, the compounds having such skeletons have ahigh hole-transport property to contribute to a reduction in drivingvoltage.

The light-emitting layer 130 and the light-emitting layer 135 can have astructure in which two or more layers are stacked. For example, in thecase where the light-emitting layer 130 or the light-emitting layer 135is formed by stacking a first light-emitting layer and a secondlight-emitting layer in this order from the hole-transport layer side,the first light-emitting layer is formed using a material having ahole-transport property as the host material and the secondlight-emitting layer is formed using a material having anelectron-transport property as the host material. A light-emittingmaterial included in the first light-emitting layer may be the same asor different from a light-emitting material included in the secondlight-emitting layer. In addition, the materials may have functions ofemitting light of the same color or light of different colors. Two kindsof light-emitting materials having functions of emitting light ofdifferent colors are used for the two light-emitting layers, so thatlight of a plurality of emission colors can be obtained at the sametime. It is particularly preferable to select light-emitting materialsof the light-emitting layers so that white light can be obtained bycombining light emission from the two light-emitting layers.

The light-emitting layer 130 may include another material in addition tothe host material 132 and the guest material 131. The light-emittinglayer 135 may include another material in addition to the host material133, the host material 132, and the guest material 131.

Note that the light-emitting layers 130 and 135 can be formed by anevaporation method (including a vacuum evaporation method), an ink-jetmethod, a coating method, gravure printing, or the like. Besides theabove-mentioned materials, an inorganic compound such as a quantum dotor a high molecular compound (e.g., an oligomer, a dendrimer, and apolymer) may be used.

<<Quantum Dot>>

A quantum dot is a semiconductor nanocrystal with a size of severalnanometers to several tens of nanometers and contains approximately1×10³ to 1×10⁶ atoms. Since energy shift of quantum dots depend on theirsize, quantum dots made of the same substance emit light with differentwavelengths depending on their size; thus, emission wavelengths can beeasily adjusted by changing the size of quantum dots.

Since a quantum dot has an emission spectrum with a narrow peak,emission with high color purity can be obtained. In addition, a quantumdot is said to have a theoretical internal quantum efficiency of 100%,which far exceeds that of a fluorescent organic compound, i.e., 25%, andis comparable to that of a phosphorescent organic compound. Therefore, aquantum dot can be used as a light-emitting material to obtain alight-emitting element having high light-emitting efficiency.Furthermore, since a quantum dot which is an inorganic material has highinherent stability, a light-emitting element which is favorable also interms of lifetime can be obtained.

Examples of a material of a quantum dot include a Group 14 element, aGroup 15 element, a Group 16 element, a compound of a plurality of Group14 elements, a compound of an element belonging to any of Groups 4 to 14and a Group 16 element, a compound of a Group 2 element and a Group 16element, a compound of a Group 13 element and a Group 15 element, acompound of a Group 13 element and a Group 17 element, a compound of aGroup 14 element and a Group 15 element, a compound of a Group 11element and a Group 17 element, iron oxides, titanium oxides, spinelchalcogenides, and semiconductor clusters.

Specific examples include, but are not limited to, cadmium selenide;cadmium sulfide; cadmium telluride; zinc selenide; zinc oxide; zincsulfide; zinc telluride; mercury sulfide; mercury selenide; mercurytelluride; indium arsenide; indium phosphide; gallium arsenide; galliumphosphide; indium nitride; gallium nitride; indium antimonide; galliumantimonide; aluminum phosphide; aluminum arsenide; aluminum antimonide;lead selenide; lead telluride; lead sulfide; indium selenide; indiumtelluride; indium sulfide; gallium selenide; arsenic sulfide; arsenicselenide; arsenic telluride; antimony sulfide; antimony selenide;antimony telluride; bismuth sulfide; bismuth selenide; bismuthtelluride; silicon; silicon carbide; germanium; tin; selenium;tellurium; boron; carbon; phosphorus; boron nitride; boron phosphide;boron arsenide; aluminum nitride; aluminum sulfide; barium sulfide;barium selenide; barium telluride; calcium sulfide; calcium selenide;calcium telluride; beryllium sulfide; beryllium selenide; berylliumtelluride; magnesium sulfide; magnesium selenide; germanium sulfide;germanium selenide; germanium telluride; tin sulfide; tin selenide; tintelluride; lead oxide; copper fluoride; copper chloride; copper bromide;copper iodide; copper oxide; copper selenide; nickel oxide; cobaltoxide; cobalt sulfide; iron oxide; iron sulfide; manganese oxide;molybdenum sulfide; vanadium oxide; tungsten oxide; tantalum oxide;titanium oxide; zirconium oxide; silicon nitride; germanium nitride;aluminum oxide; barium titanate; a compound of selenium, zinc, andcadmium; a compound of indium, arsenic, and phosphorus; a compound ofcadmium, selenium, and sulfur; a compound of cadmium, selenium, andtellurium; a compound of indium, gallium, and arsenic; a compound ofindium, gallium, and selenium; a compound of indium, selenium, andsulfur; a compound of copper, indium, and sulfur; and combinationsthereof. What is called an alloyed quantum dot, whose composition isrepresented by a given ratio, may be used. For example, an alloyedquantum dot of cadmium, selenium, and sulfur is a means effective inobtaining blue light because the emission wavelength can be changed bychanging the content ratio of elements.

As the quantum dot, any of a core-type quantum dot, a core-shell quantumdot, a core-multishell quantum dot, and the like can be used. Note thatwhen a core is covered with a shell formed of another inorganic materialhaving a wider band gap, the influence of defects and dangling bondsexisting at the surface of a nanocrystal can be reduced. Since such astructure can significantly improve the quantum efficiency of lightemission, it is preferable to use a core-shell or core-multishellquantum dot. Examples of the material of a shell include zinc sulfideand zinc oxide.

Quantum dots have a high proportion of surface atoms and thus have highreactivity and easily cohere together. For this reason, it is preferablethat a protective agent be attached to, or a protective group beprovided at the surfaces of quantum dots. The attachment of theprotective agent or the provision of the protective group can preventcohesion and increase solubility in a solvent. It can also reducereactivity and improve electrical stability. Examples of the protectiveagent (or the protective group) include polyoxyethylene alkyl etherssuch as polyoxyethylene lauryl ether, polyoxyethylene stearyl ether, andpolyoxyethylene oleyl ether; trialkylphosphines such astripropylphosphine, tributylphosphine, trihexylphosphine, andtrioctylphoshine; polyoxyethylene alkylphenyl ethers such aspolyoxyethylene n-octylphenyl ether and polyoxylethylene n-nonylphenylether; tertiary amines such as tri(n-hexyl)amine, tri(n-octyl)amine, andtri(n-decyl)amine; organophosphorus compounds such as tripropylphosphineoxide, tributylphosphine oxide, trihexylphosphine oxide,trioctylphosphine oxide, and tridecylphosphine oxide; polyethyleneglycol diesters such as polyethylene glycol dilaurate and polyethyleneglycol distearate; organic nitrogen compounds such asnitrogen-containing aromatic compounds, e.g., pyridines, lutidines,collidines, and quinolones; animoalkanes such as hexylamine, octylamine,decylamine, dodecylamine, tetradecylamine, hexadecylamine, andoctadecylamine; dialkylsulfides such as dibutylsulfide;dialkylsulfoxides such as dimethylsulfoxide and dibutylsulfoxide;organic sulfur compounds such as sulfur-containing aromatic compounds,e.g., thiophene; higher fatty acids such as a palmitin acid, a stearicacid, and an oleic acid; alcohols; sorbitan fatty acid esters; fattyacid modified polyesters; tertiary amine modified polyurethanes; andpolyethyleneimines.

Since band gaps of quantum dots are increased as their size isdecreased, the size is adjusted as appropriate so that light with adesired wavelength can be obtained. Light emission from the quantum dotsis shifted to a blue color side, i.e., a high energy side, as thecrystal size is decreased; thus, emission wavelengths of the quantumdots can be adjusted over a wavelength region of a spectrum of anultraviolet region, a visible light region, and an infrared region bychanging the size of quantum dots. The range of size (diameter) ofquantum dots which is usually used is 0.5 nm to 20 nm, preferably 1 nmto 10 nm. The emission spectra are narrowed as the size distribution ofthe quantum dots gets smaller, and thus light can be obtained with highcolor purity. The shape of the quantum dots is not particularly limitedand may be spherical shape, a rod shape, a circular shape, or the like.Quantum rods which are rod-like shape quantum dots have a function ofemitting directional light; thus, quantum rods can be used as alight-emitting material to obtain a light-emitting element with higherexternal quantum efficiency.

In most organic EL elements, to improve emission efficiency,concentration quenching of the light-emitting materials is suppressed bydispersing light-emitting materials in host materials. The hostmaterials need to be materials having singlet excitation energy levelsor triplet excitation energy levels higher than or equal to those of thelight-emitting materials. In the case of using blue phosphorescentmaterials as light-emitting materials, it is particularly difficult todevelop host materials which have triplet excitation energy levelshigher than or equal to those of the blue phosphorescent materials andwhich are excellent in terms of a lifetime. Even when a light-emittinglayer is composed of quantum dots and made without a host material, thequantum dots enable emission efficiency to be ensured; thus, alight-emitting element which is favorable in terms of a lifetime can beobtained. In the case where the light-emitting layer is composed ofquantum dots, the quantum dots preferably have core-shell structures(including core-multishell structures).

In the case of using quantum dots as the light-emitting material in thelight-emitting layer, the thickness of the light-emitting layer is setto 3 nm to 100 nm, preferably 10 nm to 100 nm, and the light-emittinglayer is made to contain 1 volume % to 100 volume % of the quantum dots.Note that it is preferable that the light-emitting layer be composed ofthe quantum dots. To form a light-emitting layer in which the quantumdots are dispersed as light-emitting materials in host materials, thequantum dots may be dispersed in the host materials, or the hostmaterials and the quantum dots may be dissolved or dispersed in anappropriate liquid medium, and then a wet process (e.g., a spin coatingmethod, a casting method, a die coating method, blade coating method, aroll coating method, an ink-jet method, a printing method, a spraycoating method, a curtain coating method, or a Langmuir-Blodgett method)may be employed. For a light-emitting layer containing a phosphorescentmaterial, a vacuum evaporation method, as well as the wet process, canbe suitably employed.

An example of the liquid medium used for the wet process is an organicsolvent of ketones such as methyl ethyl ketone and cyclohexanone; fattyacid esters such as ethyl acetate; halogenated hydrocarbons such asdichlorobenzene; aromatic hydrocarbons such as toluene, xylene,mesitylene, and cyclohexylbenzene; aliphatic hydrocarbons such ascyclohexane, decalin, and dodecane; dimethylformamide (DMF); dimethylsulfoxide (DMSO); or the like.

<<Hole-Injection Layer>>

The hole-injection layer 111 has a function of reducing a barrier forhole injection from one of the pair of electrodes (the electrode 101 orthe electrode 102) to promote hole injection and is formed using atransition metal oxide, a phthalocyanine derivative, or an aromaticamine, for example. As the transition metal oxide, molybdenum oxide,vanadium oxide, ruthenium oxide, tungsten oxide, manganese oxide, or thelike can be given. As the phthalocyanine derivative, phthalocyanine,metal phthalocyanine, or the like can be given. As the aromatic amine, abenzidine derivative, a phenylenediamine derivative, or the like can begiven. It is also possible to use a high molecular compound such aspolythiophene or polyaniline; a typical example thereof ispoly(ethylenedioxythiophene)/poly(styrenesulfonic acid), which isself-doped polythiophene.

As the hole-injection layer 111, a layer containing a composite materialof a hole-transport material and a material having a property ofaccepting electrons from the hole-transport material can also be used.Alternatively, a stack of a layer containing a material having anelectron accepting property and a layer containing a hole-transportmaterial may also be used. In a steady state or in the presence of anelectric field, electric charge can be transferred between thesematerials. As examples of the material having an electron acceptingproperty, organic acceptors such as a quinodimethane derivative, achloranil derivative, and a hexaazatriphenylene derivative can be given.A specific example is a compound having an electron-withdrawing group (ahalogen group or a cyano group), such as7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ), chloranil, or2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (abbreviation:HAT-CN). Alternatively, a transition metal oxide such as an oxide of ametal from Group 4 to Group 8 can also be used. Specifically, vanadiumoxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide,tungsten oxide, manganese oxide, rhenium oxide, or the like can be used.In particular, molybdenum oxide is preferable because it is stable inthe air, has a low hygroscopic property, and is easily handled.

A material having a property of transporting more holes than electronscan be used as the hole-transport material, and a material having a holemobility of 1×10⁻⁶ cm²/Vs or higher is preferable. Specifically, any ofthe aromatic amine, carbazole derivative, aromatic hydrocarbon, stilbenederivative, and the like described as examples of the hole-transportmaterial that can be used in the light-emitting layer can be used.Furthermore, the hole-transport material may be a high molecularcompound.

<<Hole-Transport Layer>>

The hole-transport layer 112 is a layer containing a hole-transportmaterial and can be formed using any of the hole-transport materialsgiven as examples of the material of the hole-injection layer 111. Inorder that the hole-transport layer 112 has a function of transportingholes injected to the hole-injection layer 111 to the light-emittinglayer, the highest occupied molecular orbital (HOMO) level of thehole-transport layer 112 is preferably equal or close to the HOMO levelof the hole-injection layer 111.

As the hole-transport material, a substance having a hole mobility of1×10⁻⁶ cm²/Vs or higher is preferably used. Note that other than thesesubstances, any substance that has a property of transporting more holesthan electrons may be used. The layer containing a substance having ahigh hole-transport property is not limited to a single layer, and mayinclude stacked two or more layers containing the aforementionedsubstances.

<<Electron-Transport Layer>>

The electron-transport layer 118 has a function of transporting, to thelight-emitting layer, electrons injected from the other of the pair ofelectrodes (the electrode 101 or the electrode 102) through theelectron-injection layer 119. A material having a property oftransporting more electrons than holes can be used as anelectron-transport material, and a material having an electron mobilityof 1×10⁻⁶ cm²/Vs or higher is preferable. As the compound which easilyaccepts electrons (the material having an electron-transport property),a π-electron deficient heteroaromatic compound such as anitrogen-containing heteroaromatic compound, a metal complex, or thelike can be used. Specifically, a metal complex having a quinolineligand, a benzoquinoline ligand, an oxazole ligand, or a thiazoleligand, which are described as the electron-transport materials that canbe used in the light-emitting layer, can be given. In addition, anoxadiazole derivative, a triazole derivative, a benzimidazolederivative, a quinoxaline derivative, a dibenzoquinoxaline derivative, aphenanthroline derivative, a pyridine derivative, a bipyridinederivative, a pyrimidine derivative, and a triazine derivative can begiven. A substance having an electron mobility of higher than or equalto 1×10⁻⁶ cm²/Vs is preferable. It is to be noted that any substanceother than the above substances may also be used as long it is asubstance in which the electron-transport property is higher than thehole-transport property. The electron-transport layer 118 is not limitedto a single layer, and may include stacked two or more layers containingthe aforementioned substances.

Between the electron-transport layer 118 and the light-emitting layer, alayer that controls transport of electron carriers may be provided. Thelayer is formed by addition of a small amount of a substance having ahigh electron-trapping property to a material having a highelectron-transport property described above, and the layer is capable ofadjusting carrier balance by suppressing transfer of electron carriers.Such a structure is very effective in preventing a problem (such as areduction in element lifetime) caused when electrons pass through thelight-emitting layer.

An n-type compound semiconductor may also be used, and an oxide such astitanium oxide, zinc oxide, silicon oxide, tin oxide, tungsten oxide,tantalum oxide, barium titanate, barium zirconate, zirconium oxide,hafnium oxide, aluminum oxide, yttrium oxide, or zirconium silicate; anitride such as silicon nitride; cadmium sulfide; zinc selenide; or zincsulfide can be used, for example.

<<Electron-Injection Layer>>

The electron-injection layer 119 has a function of reducing a barrierfor electron injection from the electrode 102 to promote electroninjection and can be formed using a Group 1 metal or a Group 2 metal, oran oxide, a halide, or a carbonate of any of the metals, for example.Alternatively, a composite material containing an electron-transportmaterial (described above) and a material having a property of donatingelectrons to the electron-transport material can also be used. As thematerial having an electron-donating property, a Group 1 metal, a Group2 metal, an oxide of any of the metals, or the like can be given.Specifically, an alkali metal, an alkaline earth metal, or a compoundthereof, such as lithium fluoride, sodium fluoride, cesium fluoride,calcium fluoride, or lithium oxide, can be used. Alternatively, a rareearth metal compound like erbium fluoride can be used. Electride mayalso be used for the electron-injection layer 119. Examples of theelectride include a substance in which electrons are added at highconcentration to calcium oxide-aluminum oxide. The electron-injectionlayer 119 can be formed using the substance that can be used for theelectron-transport layer 118.

A composite material in which an organic compound and an electron donor(donor) are mixed may also be used for the electron-injection layer 119.Such a composite material is excellent in an electron-injection propertyand an electron-transport property because electrons are generated inthe organic compound by the electron donor. In this case, the organiccompound is preferably a material that is excellent in transporting thegenerated electrons. Specifically, the above-listed substances forforming the electron-transport layer 118 (e.g., the metal complexes andheteroaromatic compounds) can be used, for example. As the electrondonor, a substance showing an electron-donating property with respect tothe organic compound may be used. Specifically, an alkali metal, analkaline earth metal, and a rare earth metal are preferable, andlithium, sodium, cesium, magnesium, calcium, erbium, and ytterbium aregiven. In addition, an alkali metal oxide or an alkaline earth metaloxide is preferable, and lithium oxide, calcium oxide, barium oxide, andthe like are given. A Lewis base such as magnesium oxide can also beused. An organic compound such as tetrathiafulvalene (abbreviation: TTF)can also be used.

Note that the light-emitting layer, the hole-injection layer, thehole-transport layer, the electron-transport layer, and theelectron-injection layer described above can each be formed by anevaporation method (including a vacuum evaporation method), an inkjetmethod, a coating method, a gravure printing method, or the like.Besides the above-mentioned materials, an inorganic compound such as aquantum dot or a high molecular compound (e.g., an oligomer, adendrimer, and a polymer) may be used in the light-emitting layer, thehole-injection layer, the hole-transport layer, the electron-transportlayer, and the electron-injection layer.

<<Pair of Electrodes>>

The electrodes 101 and 102 function as an anode and a cathode of eachlight-emitting element. The electrodes 101 and 102 can be formed using ametal, an alloy, or a conductive compound, a mixture or a stack thereof,or the like.

One of the electrode 101 and the electrode 102 is preferably formedusing a conductive material having a function of reflecting light.Examples of the conductive material include aluminum (Al), an alloycontaining Al, and the like. Examples of the alloy containing Al includean alloy containing Al and L (L represents one or more of titanium (Ti),neodymium (Nd), nickel (Ni), and lanthanum (La)), such as an alloycontaining Al and Ti and an alloy containing Al, Ni, and La. Aluminumhas low resistance and high light reflectivity. Aluminum is included inearth's crust in large amount and is inexpensive; therefore, it ispossible to reduce costs for manufacturing a light-emitting element withaluminum. Alternatively, silver (Ag), an alloy of Ag and N (N representsone or more of yttrium (Y), Nd, magnesium (Mg), ytterbium (Yb), Al, Ti,gallium (Ga), zinc (Zn), indium (In), tungsten (W), manganese (Mn), tin(Sn), iron (Fe), Ni, copper (Cu), palladium (Pd), iridium (Ir), and gold(Au)), or the like can be used. Examples of the alloy containing silverinclude an alloy containing silver, palladium, and copper, an alloycontaining silver and copper, an alloy containing silver and magnesium,an alloy containing silver and nickel, an alloy containing silver andgold, an alloy containing silver and ytterbium, and the like. Besides, atransition metal such as tungsten, chromium (Cr), molybdenum (Mo),copper, or titanium can be used.

Light emitted from the light-emitting layer is extracted through theelectrode 101 and/or the electrode 102. Thus, at least one of theelectrode 101 and the electrode 102 is preferably formed using aconductive material having a function of transmitting light. As theconductive material, a conductive material having a visible lighttransmittance higher than or equal to 40% and lower than or equal to100%, preferably higher than or equal to 60% and lower than or equal to100%, and a resistivity lower than or equal to 1×10⁻² f-cm can be used.

The electrodes 101 and 102 may each be formed using a conductivematerial having functions of transmitting light and reflecting light. Asthe conductive material, a conductive material having a visible lightreflectivity higher than or equal to 20% and lower than or equal to 80%,preferably higher than or equal to 40% and lower than or equal to 70%,and a resistivity lower than or equal to 1×10⁻² Ω·cm can be used. Forexample, one or more kinds of conductive metals and alloys, conductivecompounds, and the like can be used. Specifically, a metal oxide such asindium tin oxide (hereinafter, referred to as ITO), indium tin oxidecontaining silicon or silicon oxide (ITSO), indium oxide-zinc oxide(indium zinc oxide), indium oxide-tin oxide containing titanium, indiumtitanium oxide, or indium oxide containing tungsten oxide and zinc oxidecan be used. A metal thin film having a thickness that allowstransmission of light (preferably, a thickness greater than or equal to1 nm and less than or equal to 30 nm) can also be used. As the metal,Ag, an alloy of Ag and Al, an alloy of Ag and Mg, an alloy of Ag and Au,an alloy of Ag and Yb, or the like can be used.

In this specification and the like, as the material transmitting light,a material that transmits visible light and has conductivity is used.Examples of the material include, in addition to the above-describedoxide conductor typified by an ITO, an oxide semiconductor and anorganic conductor containing an organic substance. Examples of theorganic conductor containing an organic substance include a compositematerial in which an organic compound and an electron donor (donormaterial) are mixed and a composite material in which an organiccompound and an electron acceptor (acceptor material) are mixed.Alternatively, an inorganic carbon-based material such as graphene maybe used. The resistivity of the material is preferably lower than orequal to 1×10⁵ Ω·cm, further preferably lower than or equal to 1×10⁴Ω·cm.

Alternatively, the electrode 101 and/or the electrode 102 may be formedby stacking two or more of these materials.

In order to improve the light extraction efficiency, a material whoserefractive index is higher than that of an electrode having a functionof transmitting light may be formed in contact with the electrode. Thematerial may be electrically conductive or non-conductive as long as ithas a function of transmitting visible light. In addition to the oxideconductors described above, an oxide semiconductor and an organicsubstance are given as the examples of the material. Examples of theorganic substance include the materials for the light-emitting layer,the hole-injection layer, the hole-transport layer, theelectron-transport layer, and the electron-injection layer.Alternatively, an inorganic carbon-based material or a metal film thinenough to transmit light can be used. Further alternatively, a pluralityof layers each of which is formed using the material having a highrefractive index and has a thickness of several nanometers to severaltens of nanometers may be stacked.

In the case where the electrode 101 or the electrode 102 functions asthe cathode, the electrode preferably contains a material having a lowwork function (lower than or equal to 3.8 eV). The examples include anelement belonging to Group 1 or 2 of the periodic table (e.g., an alkalimetal such as lithium, sodium, or cesium, an alkaline earth metal suchas calcium or strontium, or magnesium), an alloy containing any of theseelements (e.g., Ag—Mg or Al—Li), a rare earth metal such as europium(Eu) or Yb, an alloy containing any of these rare earth metals, an alloycontaining aluminum and silver, and the like.

When the electrode 101 or the electrode 102 is used as an anode, amaterial with a high work function (4.0 eV or higher) is preferablyused.

The electrode 101 and the electrode 102 may be a stacked layer of aconductive material having a function of reflecting light and aconductive material having a function of transmitting light. In thatcase, the electrode 101 and the electrode 102 can have a function ofadjusting the optical path length so that light of a desired wavelengthemitted from each light-emitting layer resonates and is intensified,which is preferable.

As the method for forming the electrode 101 and the electrode 102, asputtering method, an evaporation method, a printing method, a coatingmethod, a molecular beam epitaxy (MBE) method, a CVD method, a pulsedlaser deposition method, an atomic layer deposition (ALD) method, or thelike can be used as appropriate.

<<Substrate>>

A light-emitting element of one embodiment of the present invention maybe formed over a substrate of glass, plastic, or the like. As the way ofstacking layers over the substrate, layers may be sequentially stackedfrom the electrode 101 side or sequentially stacked from the electrode102 side.

For the substrate over which the light-emitting element of oneembodiment of the present invention can be formed, glass, quartz,plastic, or the like can be used, for example. Alternatively, a flexiblesubstrate can be used. The flexible substrate means a substrate that canbe bent, such as a plastic substrate made of polycarbonate orpolyarylate, for example. Alternatively, a film, an inorganic vapordeposition film, or the like can be used. Another material may be usedas long as the substrate functions as a support in a manufacturingprocess of the light-emitting element or an optical element or as longas it has a function of protecting the light-emitting element or anoptical element.

In this specification and the like, a light-emitting element can beformed using any of a variety of substrates, for example. The type of asubstrate is not limited particularly. Examples of the substrate includea semiconductor substrate (e.g., a single crystal substrate or a siliconsubstrate), an SOI substrate, a glass substrate, a quartz substrate, aplastic substrate, a metal substrate, a stainless steel substrate, asubstrate including stainless steel foil, a tungsten substrate, asubstrate including tungsten foil, a flexible substrate, an attachmentfilm, and paper which include a fibrous material, a base material film,and the like. As an example of a glass substrate, a barium borosilicateglass substrate, an aluminoborosilicate glass substrate, a soda limeglass substrate, and the like can be given. Examples of the flexiblesubstrate, the attachment film, the base material film, and the like aresubstrates of plastics typified by polyethylene terephthalate (PET),polyethylene naphthalate (PEN), polyether sulfone (PES), andpolytetrafluoroethylene (PTFE). Another example is a resin such asacrylic. Furthermore, polypropylene, polyester, polyvinyl fluoride, andpolyvinyl chloride can be given as examples. Other examples arepolyamide, polyimide, aramid, epoxy, an inorganic vapor deposition film,paper, and the like.

Alternatively, a flexible substrate may be used as the substrate suchthat the light-emitting element is provided directly on the flexiblesubstrate. Further alternatively, a separation layer may be providedbetween the substrate and the light-emitting element. The separationlayer can be used when part or the whole of a light-emitting elementformed over the separation layer is separated from the substrate andtransferred onto another substrate. In such a case, the light-emittingelement can be transferred to a substrate having low heat resistance ora flexible substrate as well. For the above separation layer, a stackincluding inorganic films, which are a tungsten film and a silicon oxidefilm, or a structure in which a resin film of polyimide or the like isformed over a substrate can be used, for example.

In other words, after the light-emitting element is formed using asubstrate, the light-emitting element may be transferred to anothersubstrate. Example of the substrate to which the light-emitting elementis transferred are, in addition to the above substrates, a cellophanesubstrate, a stone substrate, a wood substrate, a cloth substrate(including a natural fiber (e.g., silk, cotton, or hemp), a syntheticfiber (e.g., nylon, polyurethane, or polyester), a regenerated fiber(e.g., acetate, cupra, rayon, or regenerated polyester), and the like),a leather substrate, a rubber substrate, and the like. When such asubstrate is used, a light-emitting element with high durability, highheat resistance, reduced weight, or reduced thickness can be formed.

The light-emitting element 150 may be formed over an electrodeelectrically connected to a field-effect transistor (FET), for example,which is formed over any of the above-described substrates. Accordingly,an active matrix display device in which the FET controls the driving ofthe light-emitting element 150 can be manufactured.

In Embodiment 1, one embodiment of the present invention has beendescribed. Other embodiments of the present invention are described inEmbodiments 2 to 9. Note that one embodiment of the present invention isnot limited thereto. That is, since various embodiments of the presentinvention are disclosed in Embodiment 1 and Embodiments 2 to 9, oneembodiment of the present invention is not limited to a specificembodiment. The example in which one embodiment of the present inventionis used in a light-emitting element is described; however, oneembodiment of the present invention is not limited thereto. For example,depending on circumstances or conditions, one embodiment of the presentinvention is not necessarily used in a light-emitting element. Oneembodiment of the present shows, but is not limited to, the example inwhich a guest material capable of converting triplet excitation energyinto light emission and at least one host material are included and inwhich the HOMO level of the guest material is higher than the HOMO levelof the host material and the energy difference between the LUMO leveland the HOMO level of the guest material is larger than the energydifference between the LUMO level and the HOMO level of the hostmaterial. Depending on circumstances or conditions, for example, theguest material in one embodiment of the present invention does notnecessarily have a function of converting the triplet excitation energyinto light emission. Alternatively, the HOMO level of the guest materialis not necessarily higher than the HOMO level of the host material.Alternatively, the energy difference between the LUMO level and the HOMOlevel of the guest material is not necessarily larger than the energydifference between the LUMO level and the HOMO level of the hostmaterial. One embodiment of the present invention shows, but is notlimited to, the example in which the host material has a difference ofgreater than 0 eV and less than or equal to 0.2 eV between the singletexcitation energy level and the triplet excitation energy level.Depending on circumstances or conditions, the host material in oneembodiment of the present invention does not necessarily have adifference of greater than 0.2 eV between the singlet excitation energylevel and the triplet excitation energy level, for example.

The structure described above in this embodiment can be combined withany of the structures described in the other embodiments as appropriate.

Embodiment 2

In this embodiment, a light-emitting element having a structuredifferent from that described in Embodiment 1 and light emissionmechanisms of the light-emitting element are described below withreference to FIGS. 5A to 5C and FIGS. 6A to 6C. In FIG. 5A and FIG. 6A,a portion having a function similar to that in FIG. 1A is represented bythe same hatch pattern as in FIG. 1A and not especially denoted by areference numeral in some cases. In addition, common reference numeralsare used for portions having similar functions, and a detaileddescription of the portions is omitted in some cases.

Structure Example 1 of Light-Emitting Element

FIG. 5A is a schematic cross-sectional view of a light-emitting element250.

The light-emitting element 250 illustrated in FIG. 5A includes aplurality of light-emitting units (a light-emitting unit 106 and alight-emitting unit 108 in FIG. 5A) between a pair of electrodes (theelectrode 101 and the electrode 102). One of light-emitting unitspreferably has the same structure as the EL layer 100. That is, it ispreferable that each of the light-emitting element 150 in FIGS. 1A and1B and the light-emitting element 152 in FIGS. 3A and 3B include onelight-emitting unit, while the light-emitting element 250 include aplurality of light-emitting units. Note that the electrode 101 functionsas an anode and the electrode 102 functions as a cathode in thefollowing description of the light-emitting element 250; however, thefunctions may be interchanged in the light-emitting element 250.

In the light-emitting element 250 illustrated in FIG. 5A, thelight-emitting unit 106 and the light-emitting unit 108 are stacked, anda charge-generation layer 115 is provided between the light-emittingunit 106 and the light-emitting unit 108. Note that the light-emittingunit 106 and the light-emitting unit 108 may have the same structure ordifferent structures. For example, it is preferable that the EL layer100 be used in the light-emitting unit 106.

The light-emitting element 250 includes a light-emitting layer 120 and alight-emitting layer 170. The light-emitting unit 106 includes thehole-injection layer 111, the hole-transport layer 112, anelectron-transport layer 113, and an electron-injection layer 114 inaddition to the light-emitting layer 170. The light-emitting unit 108includes a hole-injection layer 116, a hole-transport layer 117, anelectron-transport layer 118, and an electron-injection layer 119 inaddition to the light-emitting layer 120.

The charge-generation layer 115 may have either a structure in which anacceptor substance that is an electron acceptor is added to ahole-transport material or a structure in which a donor substance thatis an electron donor is added to an electron-transport material.Alternatively, both of these structures may be stacked.

In the case where the charge-generation layer 115 contains a compositematerial of an organic compound and an acceptor substance, the compositematerial that can be used for the hole-injection layer 111 described inEmbodiment 1 may be used for the composite material. As the organiccompound, a variety of compounds such as an aromatic amine compound, acarbazole compound, an aromatic hydrocarbon, and a high molecularcompound (such as an oligomer, a dendrimer, or a polymer) can be used. Amaterial having a hole mobility of 1×10⁻⁶ cm²/Vs or higher is preferablyused as the organic compound. Note that any other material may be usedas long as it has a property of transporting more holes than electrons.Since the composite material of an organic compound and an acceptorsubstance has excellent carrier-injection and carrier-transportproperties, low-voltage driving or low-current driving can be realized.Note that when a surface of a light-emitting unit on the anode side isin contact with the charge-generation layer 115, the charge-generationlayer 115 can also serve as a hole-injection layer or a hole-transportlayer of the light-emitting unit; thus, a hole-injection layer or ahole-transport layer need not be included in the light-emitting unit.When a surface of a light-emitting unit on the cathode side is incontact with the charge-generation layer 115, the charge-generationlayer 115 can also serve as an electron-injection layer or anelectron-transport layer of the light-emitting unit; thus, anelectron-injection layer or an electron-transport layer need not beincluded in the light-emitting unit.

The charge-generation layer 115 may have a stacked structure of a layercontaining the composite material of an organic compound and an acceptorsubstance and a layer containing another material. For example, thecharge-generation layer 115 may be formed using a combination of a layercontaining the composite material of an organic compound and an acceptorsubstance with a layer containing one compound selected from amongelectron-donating materials and a compound having a highelectron-transport property. Furthermore, the charge-generation layer115 may be formed using a combination of a layer containing thecomposite material of an organic compound and an acceptor substance witha layer containing a transparent conductive film.

The charge-generation layer 115 provided between the light-emitting unit106 and the light-emitting unit 108 may have any structure as long aselectrons can be injected to the light-emitting unit on one side andholes can be injected into the light-emitting unit on the other side inthe case where a voltage is applied between the electrode 101 and theelectrode 102. For example, in FIG. 5A, the charge-generation layer 115injects electrons into the light-emitting unit 106 and holes into thelight-emitting unit 108 when a voltage is applied such that thepotential of the electrode 101 is higher than that of the electrode 102.

Note that in terms of light extraction efficiency, the charge-generationlayer 115 preferably has a visible light transmittance (specifically, avisible light transmittance of higher than or equal to 40%). Thecharge-generation layer 115 functions even if it has lower conductivitythan the pair of electrodes (the electrodes 101 and 102).

Note that forming the charge-generation layer 115 by using any of theabove materials can suppress an increase in drive voltage caused by thestack of the light-emitting layers.

The light-emitting element having two light-emitting units has beendescribed with reference to FIG. 5A; however, a similar structure can beapplied to a light-emitting element in which three or morelight-emitting units are stacked. With a plurality of light-emittingunits partitioned by the charge-generation layer between a pair ofelectrodes as in the light-emitting element 250, it is possible toprovide a light-emitting element which can emit light having highluminance with the current density kept low and has a long lifetime. Alight-emitting element with low power consumption can be provided.

When the structures described in Embodiment 1 is used for at least oneof the plurality of units, a light-emitting element with high emissionefficiency can be provided.

It is preferable that the light-emitting layer 170 of the light-emittingunit 106 have the structure of the light-emitting layer 130 or thelight-emitting layer 135 described in Embodiment 1, in which case thelight-emitting element 250 suitably has high emission efficiency.

The light-emitting layer 120 included in the light-emitting unit 108contains a guest material 121 and a host material 122 as illustrated inFIG. 5B. Note that the guest material 121 is described below as afluorescent material.

<<Light Emission Mechanism of Light-Emitting Layer 120>>

The light emission mechanism of the light-emitting layer 120 isdescribed below.

By recombination of the electrons and holes injected from the pair ofelectrodes (the electrode 101 and the electrode 102) or thecharge-generation layer in the light-emitting layer 120, excitons areformed. Because the amount of the host material 122 is larger than thatof the guest material 121, the host material 122 is brought into anexcited state by the exciton generation.

Note that the term “exciton” refers to a carrier (electron and hole)pair. Since excitons have energy, a material where excitons aregenerated is brought into an excited state.

In the case where the formed excited state of the host material 122 is asinglet excited state, singlet excitation energy transfers from the S1level of the host material 122 to the S1 level of the guest material121, thereby forming the singlet excited state of the guest material121.

Since the guest material 121 is a fluorescent material, when a singletexcited state is formed in the guest material 121, the guest material121 immediately emits light. To obtain high light emission efficiency inthis case, the fluorescence quantum yield of the guest material 121 ispreferably high. The same can apply to a case where a singlet excitedstate is formed by recombination of carriers in the guest material 121.

Next, a case where recombination of carriers forms a triplet excitedstate of the host material 122 is described. The correlation of energylevels of the host material 122 and the guest material 121 in this caseis shown in FIG. 5C. The following explains what terms and signs in FIG.5C represent. Note that because it is preferable that the T1 level ofthe host material 122 be lower than the T1 level of the guest material121, FIG. 5C shows this preferable case. However, the T1 level of thehost material 122 may be higher than the T1 level of the guest material121.

Guest (121): the guest material 121 (the fluorescent material);

Host (122): the host material 122;

S_(FG): the S1 level of the guest material 121 (the fluorescentmaterial);

T_(FG): the T1 level of the guest material 121 (the fluorescentmaterial);

S_(FH): the S1 level the host material 122; and

T_(FH): the T1 level of the host material 122.

As illustrated in FIG. 5C, triplet-triplet annihilation (TTA) occurs,that is, triplet excitons formed by carrier recombination interact witheach other, and excitation energy is transferred and spin angularmomenta are exchanged; as a result, a reaction in which the tripletexcitons are converted into singlet exciton having energy of the S1level of the host material 122 (S_(FH)) (see TTA in FIG. 5C). Thesinglet excitation energy of the host material 122 is transferred fromS_(FH) to the S1 level of the guest material 121 (S_(FG)) having a lowerenergy than S_(FH) (see Route E₅ in FIG. 5C), and a singlet excitedstate of the guest material 121 is formed, whereby the guest material121 emits light.

Note that in the case where the density of triplet excitons in thelight-emitting layer 120 is sufficiently high (e.g., 1×10⁻¹² cm⁻³ orhigher), only the reaction of two triplet excitons close to each othercan be considered whereas deactivation of a single triplet exciton canbe ignored.

In the case where a triplet excited state of the guest material 121 isformed by carrier recombination, the triplet excited state of the guestmaterial 121 is thermally deactivated and is difficult to use for lightemission. However, in the case where the T1 level of the host material122 (T_(FH)) is lower than the T1 level of the guest material 121(T_(FG)), the triplet excitation energy of the guest material 121 can betransferred from the T1 level of the guest material 121 (T_(FG)) to theT1 level of the host material 122 (T_(FH)) (see Route E₆ in FIG. 5C) andthen is utilized for TTA.

In other words, the host material 122 preferably has a function ofconverting triplet excitation energy into singlet excitation energy bycausing TTA, so that the triplet excitation energy generated in thelight-emitting layer 120 can be partly converted into singlet excitationenergy by TTA in the host material 122. The singlet excitation energycan be transferred to the guest material 121 and extracted asfluorescence. In order to achieve this, the S1 level of the hostmaterial 122 (S_(FH)) is preferably higher than the S1 level of theguest material 121 (S_(FG)). In addition, the T1 level of the hostmaterial 122 (T_(FH)) is preferably lower than the T1 level of the guestmaterial 121 (T_(FG)).

Note that particularly in the case where the T1 level of the guestmaterial 121 (T_(FG)) is lower than the T1 level of the host material122 (T_(FH)), the weight ratio of the guest material 121 to the hostmaterial 122 is preferably low. Specifically, the weight ratio of theguest material 121 to the host material 122 is preferably greater than 0and less than or equal to 0.05, in which case the probability of carrierrecombination in the guest material 121 can be reduced. In addition, theprobability of energy transfer from the T1 level of the host material122 (T_(FH)) to the T1 level of the guest material 121 (T_(FG)) can bereduced.

Note that the host material 122 may be composed of a single compound ora plurality of compounds.

In the case where the light-emitting units 106 and 108 contain guestmaterials with different emission colors, light emitted from thelight-emitting layer 120 preferably has a peak on the shorter wavelengthside than light emitted from the light-emitting layer 170. The luminanceof a light-emitting element using a material having a high tripletexcited energy level tends to degrade quickly. TTA is utilized in thelight-emitting layer emitting light with a short wavelength so that alight-emitting element with less degradation of luminance can beprovided.

Structure Example 2 of Light-Emitting Element

FIG. 6A is a schematic cross-sectional view of a light-emitting element252.

The light-emitting element 252 illustrated in FIG. 6A includes, like thelight-emitting element 250 described above, a plurality oflight-emitting units (the light-emitting unit 106 and a light-emittingunit 110 in FIG. 6A) between a pair of electrodes (the electrode 101 andthe electrode 102). At least one of the light-emitting units has astructure similar to that of the EL layer 100. Note that thelight-emitting unit 106 and the light-emitting unit 110 may have thesame structure or different structures.

In the light-emitting element 252 illustrated in FIG. 6A, thelight-emitting unit 106 and the light-emitting unit 110 are stacked, anda charge-generation layer 115 is provided between the light-emittingunit 106 and the light-emitting unit 110. For example, it is preferablethat the EL layer 100 be used in the light-emitting unit 106.

The light-emitting element 252 includes a light-emitting layer 140 andthe light-emitting layer 170. The light-emitting unit 106 includes thehole-injection layer 111, the hole-transport layer 112, anelectron-transport layer 113, and an electron-injection layer 114 inaddition to the light-emitting layer 170. The light-emitting unit 110includes a hole-injection layer 116, a hole-transport layer 117, anelectron-transport layer 118, and an electron-injection layer 119 inaddition to the light-emitting layer 140.

When the structure described in Embodiment 1 is used for at least one ofthe plurality of units, a light-emitting element with high emissionefficiency can be provided.

The light-emitting layer of the light-emitting unit 110 preferablyincludes a phosphorescent material. In other words, it is preferablethat the light-emitting layer 140 included in the light-emitting unit110 include a phosphorescent material, and the light-emitting layer 170included in the light-emitting unit 106 have the structure of thelight-emitting layer 130 or the light-emitting layer 135 described inEmbodiment 1. A structure example of the light-emitting element 252 inthis case is described below.

The light-emitting layer 140 included in the light-emitting unit 110includes a guest material 141 and a host material 142 as illustrated inFIG. 6B. The host material 142 includes an organic compound 142_1 and anorganic compound 142_2. In the following description, the guest material141 included in the light-emitting layer 140 is a phosphorescentmaterial.

<<Light Emission Mechanism of Light-Emitting Layer 140>>

Next, the light emission mechanism of the light-emitting layer 140 isdescribed below.

The organic compound 142_1 and the organic compound 142_2 which areincluded in the light-emitting layer 140 form an exciplex.

Although it is acceptable as long as the combination of the organiccompound 142_1 and the organic compound 142_2 can form an exciplex, itis preferable that one of them be a compound having a hole-transportproperty and the other be a compound having an electron-transportproperty.

FIG. 6C shows a correlation between the energy levels of the organiccompound 142_1, the organic compound 142_2, and the guest material 141in the light-emitting layer 140. The following explains what terms andnumerals in FIG. 6C represent:

Guest (141): the guest material 141 (phosphorescent material);

Host (142_1): the organic compound 142_1 (host material);

Host (142_2): the organic compound 142_2 (host material);

T_(PG): a T1 level of the guest material 141 (phosphorescent material);

S_(PH1): an S1 level of the organic compound 142_1 (host material);

T_(PH1): a T1 level of the organic compound 142_1 (host material);

S_(PH2): an S1 level of the organic compound 142_2 (host material);

T_(PH2): a T1 level of the organic compound 142_2 (host material);

S_(PE): an S1 level of the exciplex; and

T_(PE): a T1 level of the exciplex.

The organic compound 142_1 and the organic compound 142_2 form anexciplex, and the S1 level (S_(PE)) and the T1 level (T_(PE)) of theexciplex are energy levels adjacent to each other (see Route E₇ in FIG.6C).

One of the organic compound 142_1 and the organic compound 142_2receives a hole and the other receives an electron to readily form anexciplex. Alternatively, when one of the organic compounds is broughtinto an excited state, the other immediately interacts with the one toform an exciplex. Consequently, most excitons in the light-emittinglayer 140 exist as exciplexes. Because the excitation energy levels(S_(PE) and T_(PE)) of the exciplex are lower than the S1 levels(S_(PH1) and S_(PH2)) of the host materials (the organic compounds 142_1and 142_2) that form the exciplex, the excited state of the hostmaterial 142 can be formed with lower excitation energy. This can reducethe drive voltage of the light emitting element.

Both energies of S_(PE) and T_(PE) of the exciplex are then transferredto the T1 level of the guest material 141 (the phosphorescent material);thus, light emission is obtained (see Routes E₈ and E₉ in FIG. 6C).

Furthermore, the T1 level (T_(PE)) of the exciplex is preferably higherthan the T1 level (T_(PG)) of the guest material 141. Thus, the singletexcitation energy and the triplet excitation energy of the formedexciplex can be transferred from the S1 level (S_(PE)) and the T1 level(T_(PE)) of the exciplex to the T1 level (T_(PG)) of the guest material141.

Note that in order to efficiently transfer excitation energy from theexciplex to the guest material 141, the T1 level (T_(PE)) of theexciplex is preferably lower than or equal to the T1 levels (T_(PH1) andT_(PH2)) of the organic compounds (the organic compound 142_1 and theorganic compound 142_2) which form the exciplex. Thus, quenching of thetriplet excitation energy of the exciplex due to the organic compounds(the organic compounds 142_1 and 142_2) is less likely to occur,resulting in efficient energy transfer from the exciplex to the guestmaterial 141.

In order to efficiently form an exciplex by the organic compound 142_1and the organic compound 142_2, it is preferable to satisfy thefollowing: the HOMO level of one of the organic compound 142_1 and theorganic compound 142_2 is higher than that of the other and the LUMOlevel of the one of the organic compound 142_1 and the organic compound142_2 is higher than that of the other. For example, when the organiccompound 142_1 has a hole-transport property and the organic compound142_2 has an electron-transport property, it is preferable that the HOMOlevel of the organic compound 142_1 be higher than the HOMO level of theorganic compound 142_2 and the LUMO level of the organic compound 1421be higher than the LUMO level of the organic compound 142_2.Alternatively, when the organic compound 142_2 has a hole-transportproperty and the organic compound 142_1 has an electron-transportproperty, it is preferable that the HOMO level of the organic compound142_2 be higher than the HOMO level of the organic compound 142_1 andthe LUMO level of the organic compound 142_2 be higher than the LUMOlevel of the organic compound 142_1. Specifically, the energy differencebetween the HOMO level of the organic compound 142_1 and the HOMO levelof the organic compound 142_2 is preferably greater than or equal to0.05 eV, further preferably greater than or equal to 0.1 eV, and stillfurther preferably greater than or equal to 0.2 eV. Alternatively, theenergy difference between the LUMO level of the organic compound 142_1and the LUMO level of the organic compound 142_2 is preferably greaterthan or equal to 0.05 eV, more preferably greater than or equal to 0.1eV, and still more preferably greater than or equal to 0.2 eV.

In the case where the combination of the organic compounds 142_1 and142_2 is a combination of a compound having a hole-transport propertyand a compound having an electron-transport property, the carrierbalance can be easily controlled by adjusting the mixture ratio.Specifically, the weight ratio of the compound having a hole-transportproperty to the compound having an electron-transport property ispreferably within a range of 1:9 to 9:1. Since the carrier balance canbe easily controlled with the structure, a carrier recombination regioncan also be controlled easily.

Furthermore, the mechanism of the energy transfer process between themolecules of the host material 142 (exciplex) and the guest material 141can be described using two mechanisms, i.e., Förster mechanism(dipole-dipole interaction) and Dexter mechanism (electron exchangeinteraction), as in Embodiment 1. For Förster mechanism and Dextermechanism, Embodiment 1 can be referred to.

In order to facilitate energy transfer from the singlet excited state ofthe host material (exciplex) to the triplet excited state of the guestmaterial 141 serving as an energy acceptor, it is preferable that theemission spectrum of the exciplex overlap with the absorption band ofthe guest material 141 which is on the longest wavelength side (lowestenergy side). Thus, the efficiency of generating the triplet excitedstate of the guest material 141 can be increased.

When the light-emitting layer 140 has the above-described structure,light emission from the guest material 141 (the phosphorescent material)of the light-emitting layer 140 can be obtained efficiently.

Note that the above-described processes through Routes E₇, E₈, and E₉may be referred to as exciplex-triplet energy transfer (ExTET) in thisspecification and the like. In other words, in the light-emitting layer140, excitation energy is transferred from the exciplex to the guestmaterial 141. In this case, the efficiency of reverse intersystemcrossing from T_(PE) to S_(PE) and the emission quantum yield fromS_(PE) are not necessarily high; thus, materials can be selected from awide range of options.

Note that light emitted from the light-emitting layer 170 preferably hasa peak on the shorter wavelength side than light emitted from thelight-emitting layer 140. Since the luminance of a light-emittingelement using a phosphorescent material emitting light with a shortwavelength tends to be degraded quickly, fluorescence with a shortwavelength is employed so that a light-emitting element with lessdegradation of luminance can be provided.

Note that in each of the above-described structures, the emission colorsof the guest materials used in the light-emitting unit 106 and thelight-emitting unit 108 or in the light-emitting unit 106 and thelight-emitting unit 110 may be the same or different. In the case wherethe same guest materials emitting light of the same color are used forthe light-emitting unit 106 and the light-emitting unit 108 or for thelight-emitting unit 106 and the light-emitting unit 110, thelight-emitting element 250 and the light-emitting element 252 canexhibit high emission luminance at a small current value, which ispreferable. In the case where guest materials emitting light ofdifferent colors are used for the light-emitting unit 106 and thelight-emitting unit 108 or for the light-emitting unit 106 and thelight-emitting unit 110, the light-emitting element 250 and thelight-emitting element 252 can exhibit multi-color light emission, whichis preferable. In that case, when a plurality of light-emittingmaterials with different emission wavelengths are used in one or both ofthe light-emitting layers 120 and 170 or in one or both of thelight-emitting layers 140 and 170, lights with different emission peakssynthesize light emission from the light-emitting element 250 and thelight-emitting element 252. That is, the emission spectrum of thelight-emitting element 250 has at least two maximum values.

The above structure is also suitable for obtaining white light emission.When the light-emitting layer 120 and the light-emitting layer 170 orthe light-emitting layer 140 and the light-emitting layer 170 emit lightof complementary colors, white light emission can be obtained. It isparticularly favorable to select the guest materials so that white lightemission with high color rendering properties or light emission of atleast red, green, and blue can be obtained.

At least one of the light-emitting layers 120, 140, and 170 may bedivided into layers and each of the divided layers may contain adifferent light-emitting material. That is, at least one of thelight-emitting layers 120, 140, and 170 may consist of two or morelayers. For example, in the case where the light-emitting layer isformed by stacking a first light-emitting layer and a secondlight-emitting layer in this order from the hole-transport layer side,the first light-emitting layer is formed using a material having ahole-transport property as the host material and the secondlight-emitting layer is formed using a material having anelectron-transport property as the host material. In that case, alight-emitting material included in the first light-emitting layer maybe the same as or different from a light-emitting material included inthe second light-emitting layer. In addition, the materials may havefunctions of emitting light of the same color or light of differentcolors. White light emission with a high color rendering property thatis formed of three primary colors or four or more colors can be obtainedby using a plurality of light-emitting materials emitting light ofdifferent colors.

<Material that can be Used in Light-Emitting Layers>

Next, materials that can be used in the light-emitting layers 120, 140,and 170 are described.

<<Material that can be Used in Light-Emitting Layer 120>>

In the light-emitting layer 120, the host material 122 is present in thelargest proportion by weight, and the guest material 121 (thefluorescent material) is dispersed in the host material 122. The S1level of the host material 122 is preferably higher than the S1 level ofthe guest material 121 (the fluorescent material) while the T1 level ofthe host material 122 is preferably lower than the T1 level of the guestmaterial 121 (the fluorescent material).

In the light-emitting layer 120, the guest material 121 is preferably,but not particularly limited to, an anthracene derivative, a tetracenederivative, a chrysene derivative, a phenanthrene derivative, a pyrenederivative, a perylene derivative, a stilbene derivative, an acridonederivative, a coumarin derivative, a phenoxazine derivative, aphenothiazine derivative, or the like, and for example, any of thefollowing materials can be used.

The examples include5,6-bis[4-(10-phenyl-9-anthryl)phenyl]-2,2′-bipyridine (abbreviation:PAP2BPy), 5,6-bis[4′-(10-phenyl-9-anthryl)biphenyl-4-yl]-2,2′-bipyridine(abbreviation: PAPP2BPy),N,N′-diphenyl-N,N′-bis[4-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-diamine(abbreviation: 1,6FLPAPrn),N,N′-bis(3-methylphenyl)-N,N-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-diamine(abbreviation: 1,6mMemFLPAPrn),N,N′-bis[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-N,N′-bis(4-tert-butylphenyl)pyrene-1,6-diamine(abbreviation: 1,6tBu-FLPAPrn),N,N′-diphenyl-N,N′-bis[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-3,8-dicyclohexylpyrene-1,6-diamine(abbreviation: ch-1,6FLPAPrn),N,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: YGA2S),4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine(abbreviation: YGAPA),4-(9H-carbazol-9-yl)-4′-(9,10-diphenyl-2-anthryl)triphenylamine(abbreviation: 2YGAPPA),N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: PCAPA), perylene, 2,5,8,11-tetra(tert-butyl)perylene(abbreviation: TBP),4-(10-phenyl-9-anthryl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation:PCBAPA),N,N′-(2-tert-butylanthracene-9,10-diyldi-4,1-phenylene)bis[N,N,N-triphenyl-1,4-phenylenediamine] (abbreviation: DPABPA),N,9-diphenyl-N-[4-(9,10-diphenyl-2-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: 2PCAPPA),N-[4-(9,10-diphenyl-2-anthryl)phenyl]-N,N,N-triphenyl-1,4-phenylenediamine(abbreviation: 2DPAPPA),N,N,N′,N′,N″,N″,N′″,N′″-octaphenyldibenzo[gp]chrysene-2,7,10,15-tetraamine(abbreviation: DBC1), coumarin 30,N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCABPhA),N-(9,10-diphenyl-2-anthryl)-N,N′-N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPABPhA),9,10-bis(1,1′-biphenyl-2-yl)-N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylanthracen-2-amine(abbreviation: 2YGABPhA), N,N,9-triphenylanthracen-9-amine(abbreviation: DPhAPhA), coumarin 6, coumarin 545T,N,N′-diphenylquinacridone (abbreviation: DPQd), rubrene,2,8-di-tert-butyl-5,11-bis(4-tert-butylphenyl)-6,12-diphenyltetracene(abbreviation: TBRb), Nile red,5,12-bis(1,1′-biphenyl-4-yl)-6,11-diphenyltetracene (abbreviation: BPT),2-(2-{2-[4-(dimethylamino)phenyl]ethenyl}-6-methyl-4H-pyran-4-ylidene)propanedinitrile(abbreviation: DCM1),2-{2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[j]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCM2),N,N,N′,N′-tetrakis(4-methylphenyl)tetracene-5,11-diamine (abbreviation:p-mPhTD),7,14-diphenyl-N,N,N′,N′-tetrakis(4-methylphenyl)acenaphtho[1,2-a]fluoranthene-3,10-diamine(abbreviation: p-mPhAFD),2-{2-isopropyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[j]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCJTI),2-{2-tert-butyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[j]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCJTB),2-(2,6-bis{2-[4-(dimethylamino)phenyl]ethenyl}-4H-pyran-4-ylidene)propanedinitrile(abbreviation: BisDCM),2-{2,6-bis[2-(8-methoxy-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[j]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile (abbreviation: BisDCJTM),and 5,10,15,20-tetraphenylbisbenzo[5,6]indeno[1,2,3-cd:1′,2′,3′-lm]perylene.

Although there is no particular limitation on a material that can beused as the host material 122 in the light-emitting layer 120, any ofthe following materials can be used, for example: metal complexes suchas tris(8-quinolinolato)aluminum(III) (abbreviation: Alq),tris(4-methyl-8-quinolinolato)aluminum(III) (abbreviation: Almq₃),bis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato) (4-phenylphenolato)aluminum(III)(abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq),bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), andbis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: ZnBTZ);heterocyclic compounds such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ),2,2′,2″-(1,3,5-benzenetriyl)-tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP), and9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole (abbreviation:COll); and aromatic amine compounds such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB orα-NPD),N,N-bis(3-methylphenyl)-N,N-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), and4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB). In addition, condensed polycyclic aromaticcompounds such as anthracene derivatives, phenanthrene derivatives,pyrene derivatives, chrysene derivatives, and dibenzo[gp]chrysenederivatives can be given, and specific examples are9,10-diphenylanthracene (abbreviation: DPAnth),N,N-diphenyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: CzA1PA), 4-(10-phenyl-9-anthryl)triphenylamine(abbreviation: DPhPA),4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine(abbreviation: YGAPA),N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: PCAPA),N,9-diphenyl-N-{4-[4-(10-phenyl-9-anthryl)phenyl]phenyl}-9H-carbazol-3-amine(abbreviation: PCAPBA),N,9-diphenyl-N-(9,10-diphenyl-2-anthryl)-9H-carbazol-3-amine(abbreviation: 2PCAPA), 6,12-dimethoxy-5,11-diphenylchrysene,N,N,N′,N′,N″,N″,N′″,N′″-octaphenyldibenzo[gp]chrysene-2,7,10,15-tetramine(abbreviation: DBC1), 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: CzPA),3,6-diphenyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: DPCzPA), 9,10-bis(3,5-diphenylphenyl)anthracene(abbreviation: DPPA), 9,10-di(2-naphthyl)anthracene (abbreviation: DNA),2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA),9,9′-bianthryl (abbreviation: BANT),9,9′-(stilbene-3,3′-diyl)diphenanthrene (abbreviation: DPNS),9,9′-(stilbene-4,4′-diyl)diphenanthrene (abbreviation: DPNS2),1,3,5-tri(1-pyrenyl)benzene (abbreviation: TPB3), and the like. One ormore substances having a wider energy gap than the guest material 121 ispreferably selected from these substances and known substances.

The light-emitting layer 120 can have a structure in which two or morelayers are stacked. For example, in the case where the light-emittinglayer 120 is formed by stacking a first light-emitting layer and asecond light-emitting layer in this order from the hole-transport layerside, the first light-emitting layer is formed using a substance havinga hole-transport property as the host material and the secondlight-emitting layer is formed using a substance having anelectron-transport property as the host material.

In the light-emitting layer 120, the host material 122 may be composedof one kind of compound or a plurality of compounds. Alternatively, thelight-emitting layer 120 may contain another material in addition to thehost material 122 and the guest material 121.

<<Material that can be Used in Light-Emitting Layer 140>>

In the light-emitting layer 140, the host material 142 is present in thelargest proportion by weight, and the guest material 141 (phosphorescentmaterial) is dispersed in the host material 142. The T1 levels of thehost materials 142 (organic compounds 142_1 and 142_2) of thelight-emitting layer 140 are preferably higher than the T1 level of theguest material 141 of the light-emitting layer 140.

Examples of the organic compound 142_1 include a zinc- or aluminum-basedmetal complex, an oxadiazole derivative, a triazole derivative, abenzimidazole derivative, a quinoxaline derivative, a dibenzoquinoxalinederivative, a dibenzothiophene derivative, a dibenzofuran derivative, apyrimidine derivative, a triazine derivative, a pyridine derivative, abipyridine derivative, and a phenanthroline derivative. Other examplesare an aromatic amine and a carbazole derivative. Specifically, theelectron-transport material and the hole-transport material described inEmbodiment 1 can be used.

As the organic compound 142_2, a substance which can form an exciplextogether with the organic compound 142_1 is preferably used.Specifically, the electron-transport material and the hole-transportmaterial described in Embodiment 1 can be used. In that case, it ispreferable that the organic compound 142_1, the organic compound 142_2,and the guest material 141 (phosphorescent material) be selected suchthat the emission peak of the exciplex formed by the organic compound142_1 and the organic compound 142_2 overlaps with an absorption band,specifically an absorption band on the longest wavelength side, of atriplet metal to ligand charge transfer (MLCT) transition of the guestmaterial 141 (phosphorescent material). This makes it possible toprovide a light-emitting element with drastically improved emissionefficiency. Note that in the case where a thermally activated delayedfluorescence material is used instead of the phosphorescent material, itis preferable that the absorption band on the longest wavelength side bea singlet absorption band.

As the guest material 141 (phosphorescent material), an iridium-,rhodium-, or platinum-based organometallic complex or metal complex canbe used; in particular, an organoiridium complex such as aniridium-based ortho-metalated complex is preferable. As anortho-metalated ligand, a 4H-triazole ligand, a 1H-triazole ligand, animidazole ligand, a pyridine ligand, a pyrimidine ligand, a pyrazineligand, an isoquinoline ligand, and the like can be given. As the metalcomplex, a platinum complex having a porphyrin ligand and the like canbe given. Specifically, the material described in Embodiment 1 as anexample of the guest material 131 can be used.

As the light-emitting material included in the light-emitting layer 140,any material can be used as long as the material can convert the tripletexcitation energy into light emission. As an example of the materialthat can convert the triplet excitation energy into light emission, athermally activated delayed fluorescent material can be given inaddition to a phosphorescent material. Therefore, it is acceptable thatthe “phosphorescent material” in the description is replaced with the“thermally activated delayed fluorescence material”.

The material that exhibits thermally activated delayed fluorescence maybe a material that can form a singlet excited state from a tripletexcited state by reverse intersystem crossing or may be a combination ofa plurality of materials which form an exciplex.

In the case where the material exhibiting thermally activated delayedfluorescence is formed of one kind of material, any of the thermallyactivated delayed fluorescent materials described in Embodiment 1 can bespecifically used.

In the case where the thermally activated delayed fluorescent materialis used as the host material, it is preferable to use a combination oftwo kinds of compounds which form an exciplex. In this case, it isparticularly preferable to use the above-described combination of acompound which easily accepts electrons and a compound which easilyaccepts holes, which forms an exciplex.

<<Material that can be Used in Light-Emitting Layer 170>>

As a material that can be used for the light-emitting layer 170, amaterial that can be used for the light-emitting layer in Embodiment 1can be used, so that a light-emitting element with high emissionefficiency can be formed.

There is no limitation on the emission colors of the light-emittingmaterials contained in the light-emitting layers 120, 140, and 170, andthey may be the same or different. Light emitted from the light-emittingmaterials is mixed and extracted out of the element; therefore, forexample, in the case where their emission colors are complementarycolors, the light-emitting element can emit white light. Inconsideration of the reliability of the light-emitting element, thewavelength of the emission peak of the light-emitting material containedin the light-emitting layer 120 is preferably shorter than that of thelight-emitting material contained in the light-emitting layer 170.

Note that the light-emitting units 106, 108, and 110, and thecharge-generation layer 115 can be formed by an evaporation method(including a vacuum evaporation method), an ink-jet method, a coatingmethod, gravure printing, or the like.

The structure described in this embodiment can be used in combinationwith any of the structures described in the other embodiments asappropriate.

Embodiment 3

In this embodiment, examples of light-emitting elements havingstructures different from those described in Embodiments 1 and 2 aredescribed below with reference to FIGS. 7A and 7B, FIGS. 8A and 8B,FIGS. 9A to 9C, and FIGS. 10A to 10C.

Structure Example 1 of Light-Emitting Element

FIGS. 7A and 7B are cross-sectional views each illustrating alight-emitting element of one embodiment of the present invention. InFIGS. 7A and 7B, a portion having a function similar to that in FIG. 1Ais represented by the same hatch pattern as in FIG. 1A and notespecially denoted by a reference numeral in some cases. In addition,common reference numerals are used for portions having similarfunctions, and a detailed description of the portions is omitted in somecases.

Light-emitting elements 260 a and 260 b in FIGS. 7A and 7B may have abottom-emission structure in which light is extracted through thesubstrate 200 or may have a top-emission structure in which lightemitted from the light-emitting element is extracted in the directionopposite to the substrate 200. However, one embodiment of the presentinvention is not limited to this structure, and a light-emitting elementhaving a dual-emission structure in which light emitted from thelight-emitting element is extracted in both top and bottom directions ofthe substrate 200 may be used.

In the case where the light-emitting elements 260 a and 260 b each havea bottom emission structure, the electrode 101 preferably has a functionof transmitting light and the electrode 102 preferably has a function ofreflecting light. Alternatively, in the case where the light-emittingelements 260 a and 260 b each have a top emission structure, theelectrode 101 preferably has a function of reflecting light and theelectrode 102 preferably has a function of transmitting light.

The light-emitting elements 260 a and 260 b each include the electrode101 and the electrode 102 over the substrate 200. Between the electrodes101 and 102, a light-emitting layer 123B, a light-emitting layer 123G,and a light-emitting layer 123R are provided. The hole-injection layer111, the hole-transport layer 112, the electron-transport layer 118, andthe electron-injection layer 119 are also provided.

The light-emitting element 260 b includes, as part of the electrode 101,a conductive layer 101 a, a conductive layer 101 b over the conductivelayer 101 a, and a conductive layer 101 c under the conductive layer 101a. In other words, the light-emitting element 260 b includes theelectrode 101 having a structure in which the conductive layer 101 a issandwiched between the conductive layer 101 b and the conductive layer101 c.

In the light-emitting element 260 b, the conductive layer 101 b and theconductive layer 101 c may be formed of different materials or the samematerial. The electrode 101 preferably has a structure in which theconductive layer 101 a is sandwiched by the layers formed of the sameconductive material, in which case patterning by etching in the processfor forming the electrode 101 can be performed easily.

In the light-emitting element 260 b, the electrode 101 may include oneof the conductive layer 101 b and the conductive layer 101 c.

For each of the conductive layers 101 a, 101 b, and 101 c, which areincluded in the electrode 101, the structure and materials of theelectrode 101 or 102 described in Embodiment 1 can be used.

In FIGS. 7A and 7B, a partition wall 145 is provided between a region221B, a region 221G, and a region 221R, which are sandwiched between theelectrode 101 and the electrode 102. The partition wall 145 has aninsulating property. The partition wall 145 covers end portions of theelectrode 101 and has openings overlapping with the electrode. With thepartition wall 145, the electrode 101 provided over the substrate 200 inthe regions can be divided into island shapes.

Note that the light-emitting layer 123B and the light-emitting layer123G may overlap with each other in a region where they overlap with thepartition wall 145. The light-emitting layer 123G and the light-emittinglayer 123R may overlap with each other in a region where they overlapwith the partition wall 145. The light-emitting layer 123R and thelight-emitting layer 123B may overlap with each other in a region wherethey overlap with the partition wall 145.

The partition wall 145 has an insulating property and is formed using aninorganic or organic material. Examples of the inorganic materialinclude silicon oxide, silicon oxynitride, silicon nitride oxide,silicon nitride, aluminum oxide, and aluminum nitride. Examples of theorganic material include photosensitive resin materials such as anacrylic resin and a polyimide resin.

Note that a silicon oxynitride film refers to a film in which theproportion of oxygen is higher than that of nitrogen. The siliconoxynitride film preferably contains oxygen, nitrogen, silicon, andhydrogen in the ranges of 55 atomic % to 65 atomic %, 1 atomic % to 20atomic %, 25 atomic % to 35 atomic %, and 0.1 atomic % to 10 atomic %,respectively. A silicon nitride oxide film refers to a film in which theproportion of nitrogen is higher than that of oxygen. The siliconnitride oxide film preferably contains nitrogen, oxygen, silicon, andhydrogen in the ranges of 55 atomic % to 65 atomic %, 1 atomic % to 20atomic %, 25 atomic % to 35 atomic %, and 0.1 atomic % to 10 atomic %,respectively.

The light-emitting layers 123R, 123G, and 123B preferably containlight-emitting materials having functions of emitting light of differentcolors. For example, when the light-emitting layer 123R contains alight-emitting material having a function of emitting red, the region221R emits red light. When the light-emitting layer 123G contains alight-emitting material having a function of emitting green, the region221G emits green light. When the light-emitting layer 123B contains alight-emitting material having a function of emitting blue, the region221B emits blue light. The light-emitting element 260 a or 260 b havingsuch a structure is used in a pixel of a display device, whereby afull-color display device can be fabricated. The thicknesses of thelight-emitting layers may be the same or different.

One or more of the light-emitting layer 123B, the light-emitting layer123G, and the light-emitting layer 123R preferably have at least one ofthe structures of the light-emitting layers 130 and 135 described inEmbodiment 1. In that case, a light-emitting element with high emissionefficiency can be fabricated.

One or more of the light-emitting layers 123B, 123G, and 123R mayinclude two or more stacked layers.

When at least one light-emitting layer includes the light-emitting layerdescribed in Embodiments 1 and 2 and the light-emitting element 260 a or260 b including the light-emitting layer is used in pixels in a displaydevice, a display device with high emission efficiency can befabricated. The display device including the light-emitting element 260a or 260 b can thus have reduced power consumption.

By providing an optical element (e.g., a color filter, a polarizingplate, and an anti-reflection film) on the light extraction side of theelectrode through which light is extracted, the color purity of each ofthe light-emitting elements 260 a and 260 b can be improved. Therefore,the color purity of a display device including the light-emittingelement 260 a or 260 b can be improved. Alternatively, the reflection ofexternal light by each of the light-emitting elements 260 a and 260 bcan be reduced. Therefore, the contrast ratio of a display deviceincluding the light-emitting element 260 a or 260 b can be improved.

For the other components of the light-emitting elements 260 a and 260 b,the components of the light-emitting element in Embodiments 1 and 2 maybe referred to.

Structure Example 2 of Light-Emitting Element

Next, structure examples different from the light-emitting elementsillustrated in FIGS. 7A and 7B will be described below with reference toFIGS. 8A and 8B.

FIGS. 8A and 8B are cross-sectional views of a light-emitting element ofone embodiment of the present invention. In FIGS. 8A and 8B, a portionhaving a function similar to that in FIGS. 7A and 7B is represented bythe same hatch pattern as in FIGS. 7A and 7B and not especially denotedby a reference numeral in some cases. In addition, common referencenumerals are used for portions having similar functions, and a detaileddescription of such portions is not repeated in some cases.

FIGS. 8A and 8B illustrate structure examples of a light-emittingelement including the light-emitting layer between a pair of electrodes.A light-emitting element 262 a illustrated in FIG. 8A has a top-emissionstructure in which light is extracted in a direction opposite to thesubstrate 200, and a light-emitting element 262 b illustrated in FIG. 8Bhas a bottom-emission structure in which light is extracted to thesubstrate 200 side. However, one embodiment of the present invention isnot limited to these structures and may have a dual-emission structurein which light emitted from the light-emitting element is extracted inboth top and bottom directions with respect to the substrate 200 overwhich the light-emitting element is formed.

The light-emitting elements 262 a and 262 b each include the electrode101, the electrode 102, an electrode 103, and an electrode 104 over thesubstrate 200. At least a light-emitting layer 170, a light-emittinglayer 190, and the charge-generation layer 115 are provided between theelectrode 101 and the electrode 102, between the electrode 102 and theelectrode 103, and between the electrode 102 and the electrode 104. Thehole-injection layer 111, the hole-transport layer 112, theelectron-transport layer 113, the electron-injection layer 114, thehole-injection layer 116, the hole-transport layer 117, theelectron-transport layer 118, and the electron-injection layer 119 arefurther provided.

The electrode 101 includes a conductive layer 101 a and a conductivelayer 101 b over and in contact with the conductive layer 101 a. Theelectrode 103 includes a conductive layer 103 a and a conductive layer103 b over and in contact with the conductive layer 103 a. The electrode104 includes a conductive layer 104 a and a conductive layer 104 b overand in contact with the conductive layer 104 a.

The light-emitting element 262 a illustrated in FIG. 8A and thelight-emitting element 262 b illustrated in FIG. 8B each include apartition wall 145 between a region 222B sandwiched between theelectrode 101 and the electrode 102, a region 222G sandwiched betweenthe electrode 102 and the electrode 103, and a region 222R sandwichedbetween the electrode 102 and the electrode 104. The partition wall 145has an insulating property. The partition wall 145 covers end portionsof the electrodes 101, 103, and 104 and has openings overlapping withthe electrodes. With the partition wall 145, the electrodes providedover the substrate 200 in the regions can be separated into islandshapes.

The charge-generation layer 115 can be formed with a material obtainedby adding an electron acceptor (acceptor) to a hole-transport materialor a material obtained by adding an electron donor (donor) to anelectron-transport material. Note that when the conductivity of thecharge-generation layer 115 is as high as that of the pair ofelectrodes, carriers generated in the charge-generation layer 115 mighttransfer to an adjacent pixel and light emission might occur in thepixel. In order to prevent such false light emission from an adjacentpixel, the charge-generation layer 115 is preferably formed with amaterial whose conductivity is lower than that of the pair ofelectrodes.

The light-emitting elements 262 a and 262 b each include a substrate 220provided with an optical element 224B, an optical element 224G, and anoptical element 224R in the direction in which light emitted from theregion 222B, light emitted from the region 222G, and light emitted fromthe region 222R are extracted. The light emitted from each region isemitted outside the light-emitting element through each optical element.In other words, the light from the region 222B, the light from theregion 222G, and the light from the region 222R are emitted through theoptical element 224B, the optical element 224G, and the optical element224R, respectively.

The optical elements 224B, 224G, and 224R each have a function ofselectively transmitting light of a particular color out of incidentlight. For example, the light emitted from the region 222B through theoptical element 224B is blue light, the light emitted from the region222G through the optical element 224G is green light, and the lightemitted from the region 222R through the optical element 224R is redlight.

For example, a coloring layer (also referred to as color filter), a bandpass filter, a multilayer filter, or the like can be used for theoptical elements 224R, 224G, and 224B. Alternatively, color conversionelements can be used as the optical elements. A color conversion elementis an optical element that converts incident light into light having alonger wavelength than the incident light. As the color conversionelements, quantum-dot elements can be favorably used. The usage of thequantum dot can increase color reproducibility of the display device.

One or more optical elements may be stacked over each of the opticalelements 224R, 224G, and 224B. As another optical element, a circularlypolarizing plate, an anti-reflective film, or the like can be provided,for example. A circularly polarizing plate provided on the side wherelight emitted from the light-emitting element of the display device isextracted can prevent a phenomenon in which light entering from theoutside of the display device is reflected inside the display device andreturned to the outside. An anti-reflective film can weaken externallight reflected by a surface of the display device. This leads to clearobservation of light emitted from the display device.

Note that in FIGS. 8A and 8B, blue light (B), green light (G), and redlight (R) emitted from the regions through the optical elements areschematically illustrated by arrows of dashed lines.

A light-blocking layer 223 is provided between the optical elements. Thelight-blocking layer 223 has a function of blocking light emitted fromthe adjacent regions. Note that a structure without the light-blockinglayer 223 may also be employed.

The light-blocking layer 223 has a function of reducing the reflectionof external light. The light-blocking layer 223 has a function ofpreventing mixture of light emitted from an adjacent light-emittingelement. As the light-blocking layer 223, a metal, a resin containingblack pigment, carbon black, a metal oxide, a composite oxide containinga solid solution of a plurality of metal oxides, or the like can beused.

Note that the optical element 224B and the optical element 224G mayoverlap with each other in a region where they overlap with thelight-blocking layer 223. In addition, the optical element 224G and theoptical element 224R may overlap with each other in a region where theyoverlap with the light-blocking layer 223. In addition, the opticalelement 224R and the optical element 224B may overlap with each other ina region where they overlap with the light-blocking layer 223.

As for the structures of the substrate 200 and the substrate 220provided with the optical elements, Embodiment 1 can be referred to.

Furthermore, the light-emitting elements 262 a and 262 b have amicrocavity structure.

<<Microcavity Structure>>

Light emitted from the light-emitting layer 170 and the light-emittinglayer 190 resonates between a pair of electrodes (e.g., the electrode101 and the electrode 102). The light-emitting layer 170 and thelight-emitting layer 190 are formed at such a position as to intensifythe light of a desired wavelength among light to be emitted. Forexample, by adjusting the optical length from a reflective region of theelectrode 101 to the light-emitting region of the light-emitting layer170 and the optical length from a reflective region of the electrode 102to the light-emitting region of the light-emitting layer 170, the lightof a desired wavelength among light emitted from the light-emittinglayer 170 can be intensified. By adjusting the optical length from thereflective region of the electrode 101 to the light-emitting region ofthe light-emitting layer 190 and the optical length from the reflectiveregion of the electrode 102 to the light-emitting region of thelight-emitting layer 190, the light of a desired wavelength among lightemitted from the light-emitting layer 190 can be intensified. In thecase of a light-emitting element in which a plurality of light-emittinglayers (here, the light-emitting layers 170 and 190) are stacked, theoptical lengths of the light-emitting layers 170 and 190 are preferablyoptimized.

In each of the light-emitting elements 262 a and 262 b, by adjusting thethicknesses of the conductive layers (the conductive layer 101 b, theconductive layer 103 b, and the conductive layer 104 b) in each region,the light of a desired wavelength among light emitted from thelight-emitting layers 170 and 190 can be increased. Note that thethickness of at least one of the hole-injection layer 111 and thehole-transport layer 112 or at least one of the electron-injection layer119 and the electron-transport layer 118 may differ between the regionsto increase the light emitted from the light-emitting layers 170 and190.

For example, in the case where the refractive index of the conductivematerial having a function of reflecting light in the electrodes 101 to104 is lower than the refractive index of the light-emitting layer 170or 190, the thickness of the conductive layer 101 b of the electrode 101is adjusted so that the optical length between the electrode 101 and theelectrode 102 is m_(B)λ_(B)/2 (m_(B) is a natural number and λ_(B) isthe wavelength of light intensified in the region 222B). Similarly, thethickness of the conductive layer 103 b of the electrode 103 is adjustedso that the optical length between the electrode 103 and the electrode102 is m_(G)λ_(G)/2 (m_(G) is a natural number and λ_(G) is thewavelength of light intensified in the region 222G). Furthermore, thethickness of the conductive layer 104 b of the electrode 104 is adjustedso that the optical length between the electrode 104 and the electrode102 is m_(R)λ_(R)/2 (m_(R) is a natural number and λ_(R) is thewavelength of light intensified in the region 222R).

In the case where it is difficult to precisely determine the reflectiveregions of the electrodes 101 to 104, the optical length for increasingthe intensity of light emitted from the light-emitting layer 170 or thelight-emitting layer 190 may be derived on the assumption that certainregions of the electrodes 101 to 104 are the reflective regions. In thecase where it is difficult to precisely determine the light-emittingregions of the light-emitting layer 170 and the light-emitting layer190, the optical length for increasing the intensity of light emittedfrom the light-emitting layer 170 and the light-emitting layer 190 maybe derived on the assumption that certain regions of the light-emittinglayer 170 and the light-emitting layer 190 are the light-emittingregions.

In the above manner, with the microcavity structure, in which theoptical length between the pair of electrodes in the respective regionsis adjusted, scattering and absorption of light in the vicinity of theelectrodes can be suppressed, resulting in high light extractionefficiency.

In the above structure, the conductive layers 101 b, 103 b, and 104 bpreferably have a function of transmitting light. The materials of theconductive layers 101 b, 103 b, and 104 b may be the same or different.It is preferable to use the same material for the conductive layer 101b, the conductive layer 103 b, and the conductive layer 104 b becausepatterning by etching in the formation process of the electrode 101, theelectrode 103, and the electrode 104 can be performed easily. Each ofthe conductive layers 101 b, 103 b, and 104 b may have a stackedstructure of two or more layers.

Since the light-emitting element 262 a illustrated in FIG. 8A has atop-emission structure, it is preferable that the conductive layer 101a, the conductive layer 103 a, and the conductive layer 104 a have afunction of reflecting light. In addition, it is preferable that theelectrode 102 have functions of transmitting light and reflecting light.

Since the light-emitting element 262 b illustrated in FIG. 8B has abottom-emission structure, it is preferable that the conductive layer101 a, the conductive layer 103 a, and the conductive layer 104 a havefunctions of transmitting light and reflecting light. In addition, it ispreferable that the electrode 102 have a function of reflecting light.

In each of the light-emitting elements 262 a and 262 b, the conductivelayers 101 a, 103 a, and 104 a may be formed of different materials orthe same material. When the conductive layers 101 a, 103 a, and 104 aare formed of the same material, manufacturing cost of thelight-emitting elements 262 a and 262 b can be reduced. Note that eachof the conductive layers 101 a, 103 a, and 104 a may have a stackedstructure including two or more layers.

At least one of the structures described in Embodiments 1 and 2 ispreferably used for at least one of the light-emitting layers 170 and190 included in the light-emitting elements 262 a and 262 b. In thisway, the light-emitting elements can have high emission efficiency.

Either or both of the light-emitting layers 170 and 190 may have astacked structure of two layers like the light-emitting layers 190 a and190 b, for example. Two kinds of light-emitting materials (a firstcompound and a second compound) for emitting light of different colorsare used in the two light-emitting layers, so that light of a pluralityof colors can be obtained at the same time. It is particularlypreferable to select the light-emitting materials of the light-emittinglayers so that white light can be obtained by combining light emissionsfrom the light-emitting layers 170 and 190.

Either or both of the light-emitting layers 170 and 190 may have astacked structure of three or more layers, in which a layer notincluding a light-emitting material may be included.

In the above-described manner, by using the light-emitting element 262 aor 262 b including the light-emitting layer having at least one of thestructures described in Embodiments 1 and 2 in pixels in a displaydevice, a display device with high emission efficiency can befabricated. Accordingly, the display device including the light-emittingelement 262 a or 262 b can have low power consumption.

For the other components of the light-emitting elements 262 a and 262 b,the components of the light-emitting element 260 a or 260 b or thelight-emitting element in Embodiments 1 and 2 may be referred to.

<Fabrication Method of Light-Emitting Element>

Next, a method for fabricating a light-emitting element of oneembodiment of the present invention is described below with reference toFIGS. 9A to 9C and FIGS. 10A to 10C. Here, a method for fabricating thelight-emitting element 262 a illustrated in FIG. 8A is described.

FIGS. 9A to 9C and FIGS. 10A to 10C are cross-sectional viewsillustrating a method for fabricating the light-emitting element of oneembodiment of the present invention.

The method for fabricating the light-emitting element 262 a describedbelow includes first to seventh steps.

<<First Step>>

In the first step, the electrodes (specifically the conductive layer 101a of the electrode 101, the conductive layer 103 a of the electrode 103,and the conductive layer 104 a of the electrode 104) of thelight-emitting elements are formed over the substrate 200 (see FIG. 9A).

In this embodiment, a conductive layer having a function of reflectinglight is formed over the substrate 200 and processed into a desiredshape; whereby the conductive layers 101 a, 103 a, and 104 a are formed.As the conductive layer having a function of reflecting light, an alloyfilm of silver, palladium, and copper (also referred to as an Ag—Pd—Cufilm or APC) is used. The conductive layers 101 a, 103 a, and 104 a arepreferably formed through a step of processing the same conductivelayer, because the manufacturing cost can be reduced.

Note that a plurality of transistors may be formed over the substrate200 before the first step. The plurality of transistors may beelectrically connected to the conductive layers 101 a, 103 a, and 104 a.

<<Second Step>>

In the second step, the transparent conductive layer 101 b having afunction of transmitting light is formed over the conductive layer 101 aof the electrode 101, the transparent conductive layer 103 b having afunction of transmitting light is formed over the conductive layer 103 aof the electrode 103, and the transparent conductive layer 104 b havinga function of transmitting light is formed over the conductive layer 104a of the electrode 104 (see FIG. 9B).

In this embodiment, the conductive layers 101 b, 103 b, and 104 b eachhaving a function of transmitting light are formed over the conductivelayers 101 a, 103 a, and 104 a each having a function of reflectinglight, respectively, whereby the electrode 101, the electrode 103, andthe electrode 104 are formed. As the conductive layers 101 b, 103 b, and104 b, ITSO films are used.

The conductive layers 101 b, 103 b, and 104 b having a function oftransmitting light may be formed in a plurality of steps. When theconductive layers 101 b, 103 b, and 104 b having a function oftransmitting light are formed in a plurality of steps, they can beformed to have thicknesses which enable microcavity structuresappropriate in the respective regions.

<<Third Step>>

In the third step, the partition wall 145 that covers end portions ofthe electrodes of the light-emitting element is formed (see FIG. 9C).

The partition wall 145 includes an opening overlapping with theelectrode. The conductive film exposed by the opening functions as theanode of the light-emitting element. As the partition wall 145, apolyimide-based resin is used in this embodiment.

In the first to third steps, since there is no possibility of damagingthe EL layer (a layer containing an organic compound), a variety of filmformation methods and micromachining technologies can be employed. Inthis embodiment, a reflective conductive layer is formed by a sputteringmethod, a pattern is formed over the conductive layer by a lithographymethod, and then the conductive layer is processed into an island shapeby a dry etching method or a wet etching method to form the conductivelayer 101 a of the electrode 101, the conductive layer 103 a of theelectrode 103, and the conductive layer 104 a of the electrode 104.Then, a transparent conductive film is formed by a sputtering method, apattern is formed over the transparent conductive film by a lithographymethod, and then the transparent conductive film is processed intoisland shapes by a wet etching method to form the electrodes 101, 103,and 104.

<<Fourth Step>>

In the fourth step, the hole-injection layer 111, the hole-transportlayer 112, the light-emitting layer 190, the electron-transport layer113, the electron-injection layer 114, and the charge-generation layer115 are formed (see FIG. 10A).

The hole-injection layer 111 can be formed by co-evaporating ahole-transport material and a material containing an acceptor substance.Note that a co-evaporation method is an evaporation method in which aplurality of different substances are concurrently vaporized fromrespective different evaporation sources. The hole-transport layer 112can be formed by evaporating a hole-transport material.

The light-emitting layer 190 can be formed by evaporating a guestmaterial that emits light of at least one color selected from violet,blue, blue green, green, yellow green, yellow, orange, and red. As theguest material, a fluorescent or phosphorescent organic material can beused. The structure of the light-emitting layer described in Embodiments1 and 2 is preferably employed. The light-emitting layer 190 may have atwo-layer structure. In such a case, the two light-emitting layers eachpreferably contain a light-emitting material that emits light of adifferent color.

The electron-transport layer 113 can be formed by evaporating asubstance having a high electron-transport property. Theelectron-injection layer 114 can be formed by evaporating a substancehaving a high electron-injection property.

The charge-generation layer 115 can be formed by evaporating a materialobtained by adding an electron acceptor (acceptor) to a hole-transportmaterial or a material obtained by adding an electron donor (donor) toan electron-transport material.

<<Fifth Step>>

In the fifth step, the hole-injection layer 116, the hole-transportlayer 117, the light-emitting layer 170, the electron-transport layer118, the electron-injection layer 119, and the electrode 102 are formed(see FIG. 10B).

The hole-injection layer 116 can be formed by using a material and amethod which are similar to those of the hole-injection layer 111. Thehole-transport layer 117 can be formed by using a material and a methodwhich are similar to those of the hole-transport layer 112.

The light-emitting layer 170 can be formed by evaporating a guestmaterial that emits light of at least one color selected from violet,blue, blue green, green, yellow green, yellow, orange, and red. As theguest material, a fluorescent or phosphorescent organic compound can beused. The structure of the light-emitting layer described in Embodiments1 and 2 is preferably employed. Note that at least one of thelight-emitting layer 170 and the light-emitting layer 190 preferably hasthe structure of a light-emitting layer described in Embodiment 1. Thelight-emitting layer 170 and the light-emitting layer 190 preferablyinclude light-emitting organic compounds exhibiting light of differentcolors.

The electron-transport layer 118 can be formed by using a material and amethod which are similar to those of the electron-transport layer 113.The electron-injection layer 119 can be formed by using a material and amethod which are similar to those of the electron-injection layer 114.

The electrode 102 can be formed by stacking a reflective conductive filmand a light-transmitting conductive film. The electrode 102 may have asingle-layer structure or a stacked-layer structure.

Through the above-described steps, the light-emitting element includingthe region 222B, the region 222G, and the region 222R over the electrode101, the electrode 103, and the electrode 104, respectively, are formedover the substrate 200.

<<Sixth Step>>

In the sixth step, the light-blocking layer 223, the optical element224B, the optical element 224G, and the optical element 224R are formedover the substrate 220 (see FIG. 10C).

As the light-blocking layer 223, a resin film containing black pigmentis formed in a desired region. Then, the optical element 224B, theoptical element 224G, and the optical element 224R are formed over thesubstrate 220 and the light-blocking layer 223. As the optical element224B, a resin film containing blue pigment is formed in a desiredregion. As the optical element 224G, a resin film containing greenpigment is formed in a desired region. As the optical element 224R, aresin film containing red pigment is formed in a desired region.

<<Seventh Step>>

In the seventh step, the light-emitting element formed over thesubstrate 200 is attached to the light-blocking layer 223, the opticalelement 224B, the optical element 224G, and the optical element 224Rformed over the substrate 220, and sealed with a sealant (notillustrated).

Through the above-described steps, the light-emitting element 262 aillustrated in FIG. 8A can be formed.

The structure described in this embodiment can be used in combinationwith any of the structures described in the other embodiments asappropriate.

Embodiment 4

In this embodiment, a display device of one embodiment of the presentinvention will be described below with reference to FIGS. 11A and 11B,FIGS. 12A and 12B, FIG. 13, FIGS. 14A and 14B, FIGS. 15A and 15B, FIG.16, FIGS. 17A and 17B, FIG. 18, and FIGS. 19A and 19B.

Structure Example 1 of Display Device

FIG. 11A is a top view illustrating a display device 600 and FIG. 11B isa cross-sectional view taken along the dashed-dotted line A-B and thedashed-dotted line C-D in FIG. 11A. The display device 600 includesdriver circuit portions (a signal line driver circuit portion 601 and ascan line driver circuit portion 603) and a pixel portion 602. Note thatthe signal line driver circuit portion 601, the scan line driver circuitportion 603, and the pixel portion 602 have a function of controllinglight emission from a light-emitting element.

The display device 600 also includes an element substrate 610, a sealingsubstrate 604, a sealant 605, a region 607 surrounded by the sealant605, a lead wiring 608, and an FPC 609.

Note that the lead wiring 608 is a wiring for transmitting signals to beinput to the signal line driver circuit portion 601 and the scan linedriver circuit portion 603 and for receiving a video signal, a clocksignal, a start signal, a reset signal, and the like from the FPC 609serving as an external input terminal. Although only the FPC 609 isillustrated here, the FPC 609 may be provided with a printed wiringboard (PWB).

As the signal line driver circuit portion 601, a CMOS circuit in whichan n-channel transistor 623 and a p-channel transistor 624 are combinedis formed. As the signal line driver circuit portion 601 or the scanline driver circuit portion 603, various types of circuits such as aCMOS circuit, a PMOS circuit, or an NMOS circuit can be used. Although adriver in which a driver circuit portion is formed and a pixel areformed over the same surface of a substrate in the display device ofthis embodiment, the driver circuit portion is not necessarily formedover the substrate and can be formed outside the substrate.

The pixel portion 602 includes a switching transistor 611, a currentcontrol transistor 612, and a lower electrode 613 electrically connectedto a drain of the current control transistor 612. Note that a partitionwall 614 is formed to cover end portions of the lower electrode 613. Asthe partition wall 614, for example, a positive type photosensitiveacrylic resin film can be used.

In order to obtain favorable coverage, the partition wall 614 is formedto have a curved surface with curvature at its upper or lower endportion. For example, in the case of using a positive photosensitiveacrylic as a material of the partition wall 614, it is preferable thatonly the upper end portion of the partition wall 614 have a curvedsurface with curvature (the radius of the curvature being 0.2 μm to 3μm). As the partition wall 614, either a negative photosensitive resinor a positive photosensitive resin can be used.

Note that there is no particular limitation on a structure of each ofthe transistors (the transistors 611, 612, 623, and 624). For example, astaggered transistor can be used. In addition, there is no particularlimitation on the polarity of these transistors. For these transistors,n-channel and p-channel transistors may be used, or either n-channeltransistors or p-channel transistors may be used, for example.Furthermore, there is no particular limitation on the crystallinity of asemiconductor film used for these transistors. For example, an amorphoussemiconductor film or a crystalline semiconductor film may be used.Examples of a semiconductor material include Group 14 semiconductors(e.g., a semiconductor including silicon), compound semiconductors(including oxide semiconductors), organic semiconductors, and the like.For example, it is preferable to use an oxide semiconductor that has anenergy gap of 2 eV or more, preferably 2.5 eV or more and furtherpreferably 3 eV or more, for the transistors, so that the off-statecurrent of the transistors can be reduced. Examples of the oxidesemiconductor include an In—Ga oxide and an In-M-Zn oxide (M is aluminum(Al), gallium (Ga), yttrium (Y), zirconium (Zr), lanthanum (La), cerium(Ce), tin (Sn), hafnium (Hf), or neodymium (Nd)).

An EL layer 616 and an upper electrode 617 are formed over the lowerelectrode 613. Here, the lower electrode 613 functions as an anode andthe upper electrode 617 functions as a cathode.

In addition, the EL layer 616 is formed by various methods such as anevaporation method with an evaporation mask, an ink-jet method, or aspin coating method. As another material included in the EL layer 616, alow molecular compound or a high molecular compound (including anoligomer or a dendrimer) may be used.

Note that a light-emitting element 618 is formed with the lowerelectrode 613, the EL layer 616, and the upper electrode 617. Thelight-emitting element 618 preferably has any of the structuresdescribed in Embodiments 1 to 3. In the case where the pixel portionincludes a plurality of light-emitting elements, the pixel portion mayinclude both any of the light-emitting elements described in Embodiments1 to 3 and a light-emitting element having a different structure.

When the sealing substrate 604 and the element substrate 610 areattached to each other with the sealant 605, the light-emitting element618 is provided in the region 607 surrounded by the element substrate610, the sealing substrate 604, and the sealant 605. The region 607 isfilled with a filler. In some cases, the region 607 is filled with aninert gas (nitrogen, argon, or the like) or filled with an ultravioletcurable resin or a thermosetting resin which can be used for the sealant605. For example, a polyvinyl chloride (PVC)-based resin, anacrylic-based resin, a polyimide-based resin, an epoxy-based resin, asilicone-based resin, a polyvinyl butyral (PVB)-based resin, or anethylene vinyl acetate (EVA)-based resin can be used. It is preferablethat the sealing substrate be provided with a recessed portion and adesiccant be provided in the recessed portion, in which casedeterioration due to influence of moisture can be inhibited.

An optical element 621 is provided below the sealing substrate 604 tooverlap with the light-emitting element 618. A light-blocking layer 622is provided below the sealing substrate 604. The structures of theoptical element 621 and the light-blocking layer 622 can be the same asthose of the optical element and the light-blocking layer in Embodiment3, respectively.

An epoxy-based resin or glass frit is preferably used for the sealant605. It is preferable that such a material do not transmit moisture oroxygen as much as possible. As the sealing substrate 604, a glasssubstrate, a quartz substrate, or a plastic substrate formed of fiberreinforced plastic (FRP), poly(vinyl fluoride) (PVF), polyester,acrylic, or the like can be used.

In the above-described manner, the display device including any of thelight-emitting elements and the optical elements which are described inEmbodiments 1 to 3 can be obtained.

Structure Example 2 of Display Device

Next, another example of the display device is described with referenceto FIGS. 12A and 12B and FIG. 13. Note that FIGS. 12A and 12B and FIG.13 are each a cross-sectional view of a display device of one embodimentof the present invention.

In FIG. 12A, a substrate 1001, a base insulating film 1002, a gateinsulating film 1003, gate electrodes 1006, 1007, and 1008, a firstinterlayer insulating film 1020, a second interlayer insulating film1021, a peripheral portion 1042, a pixel portion 1040, a driver circuitportion 1041, lower electrodes 1024R, 1024G, and 1024B of light-emittingelements, a partition wall 1025, an EL layer 1028, an upper electrode1026 of the light-emitting elements, a sealing layer 1029, a sealingsubstrate 1031, a sealant 1032, and the like are illustrated.

In FIG. 12A, examples of the optical elements, coloring layers (a redcoloring layer 1034R, a green coloring layer 1034G, and a blue coloringlayer 1034B) are provided on a transparent base material 1033. Further,a light-blocking layer 1035 may be provided. The transparent basematerial 1033 provided with the coloring layers and the light-blockinglayer is positioned and fixed to the substrate 1001. Note that thecoloring layers and the light-blocking layer are covered with anovercoat layer 1036. In the structure in FIG. 12A, red light, greenlight, and blue light transmit the coloring layers, and thus an imagecan be displayed with the use of pixels of three colors.

FIG. 12B illustrates an example in which, as examples of the opticalelements, the coloring layers (the red coloring layer 1034R, the greencoloring layer 1034G, and the blue coloring layer 1034B) are providedbetween the gate insulating film 1003 and the first interlayerinsulating film 1020. As in this structure, the coloring layers may beprovided between the substrate 1001 and the sealing substrate 1031.

FIG. 13 illustrates an example in which, as examples of the opticalelements, the coloring layers (the red coloring layer 1034R, the greencoloring layer 1034G, and the blue coloring layer 1034B) are providedbetween the first interlayer insulating film 1020 and the secondinterlayer insulating film 1021. As in this structure, the coloringlayers may be provided between the substrate 1001 and the sealingsubstrate 1031.

The above-described display device has a structure in which light isextracted from the substrate 1001 side where the transistors are formed(a bottom-emission structure), but may have a structure in which lightis extracted from the sealing substrate 1031 side (a top-emissionstructure).

Structure Example 3 of Display Device

FIGS. 14A and 14B are each an example of a cross-sectional view of adisplay device having a top emission structure. Note that FIGS. 14A and14B are each a cross-sectional view illustrating the display device ofone embodiment of the present invention, and the driver circuit portion1041, the peripheral portion 1042, and the like, which are illustratedin FIGS. 12A and 12B and FIG. 13, are not illustrated therein.

In this case, as the substrate 1001, a substrate that does not transmitlight can be used. The process up to the step of forming a connectionelectrode which connects the transistor and the anode of thelight-emitting element is performed in a manner similar to that of thedisplay device having a bottom-emission structure. Then, a thirdinterlayer insulating film 1037 is formed to cover an electrode 1022.This insulating film may have a planarization function. The thirdinterlayer insulating film 1037 can be formed using a material similarto that of the second interlayer insulating film, or can be formed usingany other various materials.

The lower electrodes 1024R, 1024G, and 1024B of the light-emittingelements each function as an anode here, but may function as a cathode.Further, in the case of a display device having a top-emission structureas illustrated in FIGS. 14A and 14B, the lower electrodes 1024R, 1024G,and 1024B preferably have a function of reflecting light. The upperelectrode 1026 is provided over the EL layer 1028. It is preferable thatthe upper electrode 1026 have a function of reflecting light and afunction of transmitting light and that a microcavity structure be usedbetween the upper electrode 1026 and the lower electrodes 1024R, 1024G,and 1024B, in which case the intensity of light having a specificwavelength is increased.

In the case of a top-emission structure as illustrated in FIG. 14A,sealing can be performed with the sealing substrate 1031 on which thecoloring layers (the red coloring layer 1034R, the green coloring layer1034G, and the blue coloring layer 1034B) are provided. The sealingsubstrate 1031 may be provided with the light-blocking layer 1035 whichis positioned between pixels. Note that a light-transmitting substrateis favorably used as the sealing substrate 1031.

FIG. 14A illustrates the structure provided with the light-emittingelements and the coloring layers for the light-emitting elements as anexample; however, the structure is not limited thereto. For example, asshown in FIG. 14B, a structure including the red coloring layer 1034Rand the blue coloring layer 1034B but not including a green coloringlayer may be employed to achieve full color display with the threecolors of red, green, and blue. The structure as illustrated in FIG. 14Awhere the light-emitting elements are provided with the coloring layersis effective to suppress reflection of external light. In contrast, thestructure as illustrated in FIG. 14B where the light-emitting elementsare provided with the red coloring layer and the blue coloring layer andwithout the green coloring layer is effective to reduce powerconsumption because of small energy loss of light emitted from the greenlight-emitting element.

Structure Example 4 of Display Device

Although a display device including sub-pixels of three colors (red,green, and blue) is described above, the number of colors of sub-pixelsmay be four (red, green, blue, and yellow, or red, green, blue, andwhite). FIGS. 15A and 15B, FIG. 16, and FIGS. 17A and 17B illustratestructures of display devices each including the lower electrodes 1024R,1024G, 1024B, and 1024Y. FIGS. 15A and 15B and FIG. 16 each illustrate adisplay device having a structure in which light is extracted from thesubstrate 1001 side on which transistors are formed (bottom-emissionstructure), and FIGS. 17A and 17B each illustrate a display devicehaving a structure in which light is extracted from the sealingsubstrate 1031 side (top-emission structure).

FIG. 15A illustrates an example of a display device in which opticalelements (the coloring layer 1034R, the coloring layer 1034G, thecoloring layer 1034B, and a coloring layer 1034Y) are provided on thetransparent base material 1033. FIG. 15B illustrates an example of adisplay device in which optical elements (the coloring layer 1034R, thecoloring layer 1034G, the coloring layer 1034B, and the coloring layer1034Y) are provided between the gate insulating film 1003 and the firstinterlayer insulating film 1020. FIG. 16 illustrates an example of adisplay device in which optical elements (the coloring layer 1034R, thecoloring layer 1034G, the coloring layer 1034B, and the coloring layer1034Y) are provided between the first interlayer insulating film 1020and the second interlayer insulating film 1021.

The coloring layer 1034R transmits red light, the coloring layer 1034Gtransmits green light, and the coloring layer 1034B transmits bluelight. The coloring layer 1034Y transmits yellow light or transmitslight of a plurality of colors selected from blue, green, yellow, andred. When the coloring layer 1034Y can transmit light of a plurality ofcolors selected from blue, green, yellow, and red, light released fromthe coloring layer 1034Y may be white light. Since the light-emittingelement which transmits yellow or white light has high emissionefficiency, the display device including the coloring layer 1034Y canhave lower power consumption.

In the top-emission display devices illustrated in FIGS. 17A and 17B, alight-emitting element including the lower electrode 1024Y preferablyhas a microcavity structure between the lower electrode 1024Y and theupper electrode 1026 as in the display device illustrated in FIG. 14A.In the display device illustrated in FIG. 17A, sealing can be performedwith the sealing substrate 1031 on which the coloring layers (the redcoloring layer 1034R, the green coloring layer 1034G, the blue coloringlayer 1034B, and the yellow coloring layer 1034Y) are provided.

Light emitted through the microcavity and the yellow coloring layer1034Y has an emission spectrum in a yellow region. Since yellow is acolor with a high luminosity factor, a light-emitting element emittingyellow light has high emission efficiency. Therefore, the display deviceof FIG. 17A can reduce power consumption.

FIG. 17A illustrates the structure provided with the light-emittingelements and the coloring layers for the light-emitting elements as anexample; however, the structure is not limited thereto. For example, asshown in FIG. 17B, a structure including the red coloring layer 1034R,the green coloring layer 1034G, and the blue coloring layer 1034B butnot including a yellow coloring layer may be employed to achieve fullcolor display with the four colors of red, green, blue, and yellow or ofred, green, blue, and white. The structure as illustrated in FIG. 17Awhere the light-emitting elements are provided with the coloring layersis effective to suppress reflection of external light. In contrast, thestructure as illustrated in FIG. 17B where the light-emitting elementsare provided with the red coloring layer, the green coloring layer, andthe blue coloring layer and without the yellow coloring layer iseffective to reduce power consumption because of small energy loss oflight emitted from the yellow or white light-emitting element.

Structure Example 5 of Display Device

Next, a display device of another embodiment of the present invention isdescribed with reference to FIG. 18. FIG. 18 is a cross-sectional viewtaken along the dashed-dotted line A-B and the dashed-dotted line C-D inFIG. 11A. Note that in FIG. 18, portions having functions similar tothose of portions in FIG. 11B are given the same reference numerals asin FIG. 11B, and a detailed description of the portions is omitted.

The display device 600 in FIG. 18 includes a sealing layer 607 a, asealing layer 607 b, and a sealing layer 607 c in a region 607surrounded by the element substrate 610, the sealing substrate 604, andthe sealant 605. For one or more of the sealing layer 607 a, the sealinglayer 607 b, and the sealing layer 607 c, a resin such as a polyvinylchloride (PVC) based resin, an acrylic-based resin, a polyimide-basedresin, an epoxy-based resin, a silicone-based resin, a polyvinyl butyral(PVB) based resin, or an ethylene vinyl acetate (EVA) based resin can beused. Alternatively, an inorganic material such as silicon oxide,silicon oxynitride, silicon nitride oxide, silicon nitride, aluminumoxide, or aluminum nitride can be used. The formation of the sealinglayers 607 a, 607 b, and 607 c can prevent deterioration of thelight-emitting element 618 due to impurities such as water, which ispreferable. In the case where the sealing layers 607 a, 607 b, and 607 care formed, the sealant 605 is not necessarily provided.

Alternatively, any one or two of the sealing layers 607 a, 607 b, and607 c may be provided or four or more sealing layers may be formed. Whenthe sealing layer has a multilayer structure, the impurities such aswater can be effectively prevented from entering the light-emittingelement 618 which is inside the display device from the outside of thedisplay device 600. In the case where the sealing layer has a multilayerstructure, a resin and an inorganic material are preferably stacked.

Structure Example 6 of Display Device

Although the display devices in the structure examples 1 to 4 in thisembodiment each have a structure including optical elements, oneembodiment of the present invention does not necessarily include anoptical element.

FIGS. 19A and 19B each illustrate a display device having a structure inwhich light is extracted from the sealing substrate 1031 side (atop-emission display device). FIG. 19A illustrates an example of adisplay device including a light-emitting layer 1028R, a light-emittinglayer 1028G, and a light-emitting layer 1028B. FIG. 19B illustrates anexample of a display device including a light-emitting layer 1028R, alight-emitting layer 1028G, a light-emitting layer 1028B, and alight-emitting layer 1028Y.

The light-emitting layer 1028R has a function of exhibiting red light,the light-emitting layer 1028G has a function of exhibiting green light,and the light-emitting layer 1028B has a function of exhibiting bluelight. The light-emitting layer 1028Y has a function of exhibitingyellow light or a function of exhibiting light of a plurality of colorsselected from blue, green, and red. The light-emitting layer 1028Y mayexhibit white light. Since the light-emitting element which exhibitsyellow or white light has high light emission efficiency, the displaydevice including the light-emitting layer 1028Y can have lower powerconsumption.

Each of the display devices in FIGS. 19A and 19B does not necessarilyinclude coloring layers serving as optical elements because EL layersexhibiting light of different colors are included in sub-pixels.

For the sealing layer 1029, a resin such as a polyvinyl chloride (PVC)based resin, an acrylic-based resin, a polyimide-based resin, anepoxy-based resin, a silicone-based resin, a polyvinyl butyral (PVB)based resin, or an ethylene vinyl acetate (EVA) based resin can be used.Alternatively, an inorganic material such as silicon oxide, siliconoxynitride, silicon nitride oxide, silicon nitride, aluminum oxide, oraluminum nitride can be used. The formation of the sealing layer 1029can prevent deterioration of the light-emitting element due toimpurities such as water, which is preferable.

Alternatively, the sealing layer 1029 may have a single-layer ortwo-layer structure, or four or more sealing layers may be formed as thesealing layer 1029. When the sealing layer has a multilayer structure,the impurities such as water can be effectively prevented from enteringthe inside of the display device from the outside of the display device.In the case where the sealing layer has a multilayer structure, a resinand an inorganic material are preferably stacked.

Note that the sealing substrate 1031 has a function of protecting thelight-emitting element. Thus, for the sealing substrate 1031, a flexiblesubstrate or a film can be used.

The structures described in this embodiment can be combined asappropriate with any of the other structures in this embodiment and theother embodiments.

Embodiment 5

In this embodiment, a display device including a light-emitting elementof one embodiment of the present invention will be described withreference to FIGS. 20A and 20B, FIGS. 21A and 21B, and FIGS. 22A and22B.

FIG. 20A is a block diagram illustrating the display device of oneembodiment of the present invention, and FIG. 20B is a circuit diagramillustrating a pixel circuit of the display device of one embodiment ofthe present invention.

<Description of Display Device>

The display device illustrated in FIG. 20A includes a region includingpixels of display elements (the region is hereinafter referred to as apixel portion 802), a circuit portion provided outside the pixel portion802 and including circuits for driving the pixels (the portion ishereinafter referred to as a driver circuit portion 804), circuitshaving a function of protecting elements (the circuits are hereinafterreferred to as protection circuits 806), and a terminal portion 807.Note that the protection circuits 806 are not necessarily provided.

A part or the whole of the driver circuit portion 804 is preferablyformed over a substrate over which the pixel portion 802 is formed, inwhich case the number of components and the number of terminals can bereduced. When a part or the whole of the driver circuit portion 804 isnot formed over the substrate over which the pixel portion 802 isformed, the part or the whole of the driver circuit portion 804 can bemounted by COG or tape automated bonding (TAB).

The pixel portion 802 includes a plurality of circuits for drivingdisplay elements arranged in X rows (X is a natural number of 2 or more)and Y columns (Y is a natural number of 2 or more) (such circuits arehereinafter referred to as pixel circuits 801). The driver circuitportion 804 includes driver circuits such as a circuit for supplying asignal (scan signal) to select a pixel (the circuit is hereinafterreferred to as a scan line driver circuit 804 a) and a circuit forsupplying a signal (data signal) to drive a display element in a pixel(the circuit is hereinafter referred to as a signal line driver circuit804 b).

The scan line driver circuit 804 a includes a shift register or thelike. Through the terminal portion 807, the scan line driver circuit 804a receives a signal for driving the shift register and outputs a signal.For example, the scan line driver circuit 804 a receives a start pulsesignal, a clock signal, or the like and outputs a pulse signal. The scanline driver circuit 804 a has a function of controlling the potentialsof wirings supplied with scan signals (such wirings are hereinafterreferred to as scan lines GL_1 to GL_X). Note that a plurality of scanline driver circuits 804 a may be provided to control the scan linesGL_1 to GL_X separately. Alternatively, the scan line driver circuit 804a has a function of supplying an initialization signal. Without beinglimited thereto, the scan line driver circuit 804 a can supply anothersignal.

The signal line driver circuit 804 b includes a shift register or thelike. The signal line driver circuit 804 b receives a signal (imagesignal) from which a data signal is derived, as well as a signal fordriving the shift register, through the terminal portion 807. The signalline driver circuit 804 b has a function of generating a data signal tobe written to the pixel circuit 801 which is based on the image signal.In addition, the signal line driver circuit 804 b has a function ofcontrolling output of a data signal in response to a pulse signalproduced by input of a start pulse signal, a clock signal, or the like.Furthermore, the signal line driver circuit 804 b has a function ofcontrolling the potentials of wirings supplied with data signals (suchwirings are hereinafter referred to as data lines DL_1 to DL_Y).Alternatively, the signal line driver circuit 804 b has a function ofsupplying an initialization signal. Without being limited thereto, thesignal line driver circuit 804 b can supply another signal.

The signal line driver circuit 804 b includes a plurality of analogswitches or the like, for example. The signal line driver circuit 804 bcan output, as the data signals, signals obtained by time-dividing theimage signal by sequentially turning on the plurality of analogswitches. The signal line driver circuit 804 b may include a shiftregister or the like.

A pulse signal and a data signal are input to each of the plurality ofpixel circuits 801 through one of the plurality of scan lines GLsupplied with scan signals and one of the plurality of data lines DLsupplied with data signals, respectively. Writing and holding of thedata signal to and in each of the plurality of pixel circuits 801 arecontrolled by the scan line driver circuit 804 a. For example, to thepixel circuit 801 in the m-th row and the n-th column (m is a naturalnumber of less than or equal to X, and n is a natural number of lessthan or equal to Y), a pulse signal is input from the scan line drivercircuit 804 a through the scan line GL_m, and a data signal is inputfrom the signal line driver circuit 804 b through the data line DL_n inaccordance with the potential of the scan line GL_m.

The protection circuit 806 shown in FIG. 20A is connected to, forexample, the scan line GL between the scan line driver circuit 804 a andthe pixel circuit 801. Alternatively, the protection circuit 806 isconnected to the data line DL between the signal line driver circuit 804b and the pixel circuit 801. Alternatively, the protection circuit 806can be connected to a wiring between the scan line driver circuit 804 aand the terminal portion 807. Alternatively, the protection circuit 806can be connected to a wiring between the signal line driver circuit 804b and the terminal portion 807. Note that the terminal portion 807 meansa portion having terminals for inputting power, control signals, andimage signals to the display device from external circuits.

The protection circuit 806 is a circuit that electrically connects awiring connected to the protection circuit to another wiring when apotential out of a certain range is applied to the wiring connected tothe protection circuit.

As illustrated in FIG. 20A, the protection circuits 806 are connected tothe pixel portion and the driver circuit portion 804, so that theresistance of the display device to overcurrent generated byelectrostatic discharge (ESD) or the like can be improved. Note that theconfiguration of the protection circuits 806 is not limited to that, andfor example, a configuration in which the protection circuits 806 areconnected to the scan line driver circuit 804 a or a configuration inwhich the protection circuits 806 are connected to the signal linedriver circuit 804 b may be employed. Alternatively, the protectioncircuits 806 may be configured to be connected to the terminal portion807.

In FIG. 20A, an example in which the driver circuit portion 804 includesthe scan line driver circuit 804 a and the signal line driver circuit804 b is shown; however, the structure is not limited thereto. Forexample, only the scan line driver circuit 804 a may be formed and aseparately prepared substrate where a signal line driver circuit isformed (e.g., a driver circuit substrate formed with a single crystalsemiconductor film or a polycrystalline semiconductor film) may bemounted.

Structure Example of Pixel Circuit

Each of the plurality of pixel circuits 801 in FIG. 20A can have astructure illustrated in FIG. 20B, for example.

The pixel circuit 801 illustrated in FIG. 20B includes transistors 852and 854, a capacitor 862, and a light-emitting element 872.

One of a source electrode and a drain electrode of the transistor 852 iselectrically connected to a wiring to which a data signal is supplied (adata line DL_n). A gate electrode of the transistor 852 is electricallyconnected to a wiring to which a gate signal is supplied (a scan lineGL_m).

The transistor 852 has a function of controlling whether to write a datasignal.

One of a pair of electrodes of the capacitor 862 is electricallyconnected to a wiring to which a potential is supplied (hereinafterreferred to as a potential supply line VL_a), and the other iselectrically connected to the other of the source electrode and thedrain electrode of the transistor 852.

The capacitor 862 functions as a storage capacitor for storing writtendata.

One of a source electrode and a drain electrode of the transistor 854 iselectrically connected to the potential supply line VL_a. Furthermore, agate electrode of the transistor 854 is electrically connected to theother of the source electrode and the drain electrode of the transistor852.

One of an anode and a cathode of the light-emitting element 872 iselectrically connected to a potential supply line VL_b, and the other iselectrically connected to the other of the source electrode and thedrain electrode of the transistor 854.

As the light-emitting element 872, any of the light-emitting elementsdescribed in Embodiments 1 to 3 can be used.

Note that a high power supply potential VDD is supplied to one of thepotential supply line VL_a and the potential supply line VL_b, and a lowpower supply potential VSS is supplied to the other.

In the display device including the pixel circuits 801 in FIG. 20B, thepixel circuits 801 are sequentially selected row by row by the scan linedriver circuit 804 a in FIG. 20A, for example, whereby the transistors852 are turned on and a data signal is written.

When the transistors 852 are turned off, the pixel circuits 801 in whichthe data has been written are brought into a holding state. Furthermore,the amount of current flowing between the source electrode and the drainelectrode of the transistor 854 is controlled in accordance with thepotential of the written data signal. The light-emitting element 872emits light with a luminance corresponding to the amount of flowingcurrent. This operation is sequentially performed row by row; thus, animage is displayed.

Alternatively, the pixel circuit can have a function of compensatingvariation in threshold voltages or the like of a transistor. FIGS. 21Aand 21B and FIGS. 22A and 22B illustrate examples of the pixel circuit.

The pixel circuit illustrated in FIG. 21A includes six transistors(transistors 303_1 to 303_6), a capacitor 304, and a light-emittingelement 305. The pixel circuit illustrated in FIG. 21A is electricallyconnected to wirings 301_1 to 301_5 and wirings 302_1 and 302_2. Notethat as the transistors 303_1 to 303_6, for example, p-channeltransistors can be used.

The pixel circuit shown in FIG. 21B has a configuration in which atransistor 303_7 is added to the pixel circuit shown in FIG. 21A. Thepixel circuit illustrated in FIG. 21B is electrically connected towirings 301_6 and 301_7. The wirings 3015 and 301_6 may be electricallyconnected to each other. Note that as the transistor 303_7, for example,a p-channel transistor can be used.

The pixel circuit shown in FIG. 22A includes six transistors(transistors 308_1 to 308_6), the capacitor 304, and the light-emittingelement 305. The pixel circuit illustrated in FIG. 22A is electricallyconnected to wirings 306_1 to 306_3 and wirings 307_1 to 307_3. Thewirings 306_1 and 3063 may be electrically connected to each other. Notethat as the transistors 308_1 to 308_6, for example, p-channeltransistors can be used.

The pixel circuit illustrated in FIG. 22B includes two transistors(transistors 309_1 and 309_2), two capacitors (capacitors 304_1 and304_2), and the light-emitting element 305. The pixel circuitillustrated in FIG. 22B is electrically connected to wirings 311_1 to311_3 and wirings 3121 and 312_2. With the configuration of the pixelcircuit illustrated in FIG. 22B, the pixel circuit can be driven by avoltage inputting current driving method (also referred to as CVCC).Note that as the transistors 309_1 and 309_2, for example, p-channeltransistors can be used.

A light-emitting element of one embodiment of the present invention canbe used for an active matrix method in which an active element isincluded in a pixel of a display device or a passive matrix method inwhich an active element is not included in a pixel of a display device.

In the active matrix method, as an active element (a non-linearelement), not only a transistor but also a variety of active elements(non-linear elements) can be used. For example, a metal insulator metal(MIM), a thin film diode (TFD), or the like can also be used. Sincethese elements can be formed with a smaller number of manufacturingsteps, manufacturing cost can be reduced or yield can be improved.Alternatively, since the size of these elements is small, the apertureratio can be improved, so that power consumption can be reduced andhigher luminance can be achieved.

As a method other than the active matrix method, the passive matrixmethod in which an active element (a non-linear element) is not used canalso be used. Since an active element (a non-linear element) is notused, the number of manufacturing steps is small, so that manufacturingcost can be reduced or yield can be improved. Alternatively, since anactive element (a non-linear element) is not used, the aperture ratiocan be improved, so that power consumption can be reduced or higherluminance can be achieved, for example.

The structure described in this embodiment can be used in combinationwith any of the structures described in the other embodiments asappropriate.

Embodiment 6

In this embodiment, a display device including a light-emitting elementof one embodiment of the present invention and an electronic device inwhich the display device is provided with an input device will bedescribed with reference to FIGS. 23A and 23B, FIGS. 24A to 24C, FIGS.25A and 25B, FIGS. 26A and 26B, and FIG. 27.

<Description 1 of Touch Panel>

In this embodiment, a touch panel 2000 including a display device and aninput device will be described as an example of an electronic device. Inaddition, an example in which a touch sensor is included as an inputdevice will be described.

FIGS. 23A and 23B are perspective views of the touch panel 2000. Notethat FIGS. 23A and 23B illustrate only main components of the touchpanel 2000 for simplicity.

The touch panel 2000 includes a display device 2501 and a touch sensor2595 (see FIG. 23B). The touch panel 2000 also includes a substrate2510, a substrate 2570, and a substrate 2590. The substrate 2510, thesubstrate 2570, and the substrate 2590 each have flexibility. Note thatone or all of the substrates 2510, 2570, and 2590 may be inflexible.

The display device 2501 includes a plurality of pixels over thesubstrate 2510 and a plurality of wirings 2511 through which signals aresupplied to the pixels. The plurality of wirings 2511 are led to aperipheral portion of the substrate 2510, and parts of the plurality ofwirings 2511 form a terminal 2519. The terminal 2519 is electricallyconnected to an FPC 2509(1). The plurality of wirings 2511 can supplysignals from a signal line driver circuit 2503 s(1) to the plurality ofpixels.

The substrate 2590 includes the touch sensor 2595 and a plurality ofwirings 2598 electrically connected to the touch sensor 2595. Theplurality of wirings 2598 are led to a peripheral portion of thesubstrate 2590, and parts of the plurality of wirings 2598 form aterminal. The terminal is electrically connected to an FPC 2509(2). Notethat in FIG. 23B, electrodes, wirings, and the like of the touch sensor2595 provided on the back side of the substrate 2590 (the side facingthe substrate 2510) are indicated by solid lines for clarity.

As the touch sensor 2595, a capacitive touch sensor can be used.Examples of the capacitive touch sensor are a surface capacitive touchsensor and a projected capacitive touch sensor.

Examples of the projected capacitive touch sensor are a self capacitivetouch sensor and a mutual capacitive touch sensor, which differ mainlyin the driving method. The use of a mutual capacitive type is preferablebecause multiple points can be sensed simultaneously.

Note that the touch sensor 2595 illustrated in FIG. 23B is an example ofusing a projected capacitive touch sensor.

Note that a variety of sensors that can sense proximity or touch of asensing target such as a finger can be used as the touch sensor 2595.

The projected capacitive touch sensor 2595 includes electrodes 2591 andelectrodes 2592. The electrodes 2591 are electrically connected to anyof the plurality of wirings 2598, and the electrodes 2592 areelectrically connected to any of the other wirings 2598.

The electrodes 2592 each have a shape of a plurality of quadranglesarranged in one direction with one corner of a quadrangle connected toone corner of another quadrangle as illustrated in FIGS. 23A and 23B.

The electrodes 2591 each have a quadrangular shape and are arranged in adirection intersecting with the direction in which the electrodes 2592extend.

A wiring 2594 electrically connects two electrodes 2591 between whichthe electrode is positioned. The intersecting area of the electrode 2592and the wiring 2594 is preferably as small as possible. Such a structureallows a reduction in the area of a region where the electrodes are notprovided, reducing variation in transmittance. As a result, variation inluminance of light passing through the touch sensor 2595 can be reduced.

Note that the shapes of the electrodes 2591 and the electrodes 2592 arenot limited thereto and can be any of a variety of shapes. For example,a structure may be employed in which the plurality of electrodes 2591are arranged so that gaps between the electrodes 2591 are reduced asmuch as possible, and the electrodes 2592 are spaced apart from theelectrodes 2591 with an insulating layer interposed therebetween to haveregions not overlapping with the electrodes 2591. In this case, it ispreferable to provide, between two adjacent electrodes 2592, a dummyelectrode electrically insulated from these electrodes because the areaof regions having different transmittances can be reduced.

<Description of Display Device>

Next, the display device 2501 will be described in detail with referenceto FIG. 24A. FIG. 24A corresponds to a cross-sectional view taken alongdashed-dotted line X1-X2 in FIG. 23B.

The display device 2501 includes a plurality of pixels arranged in amatrix. Each of the pixels includes a display element and a pixelcircuit for driving the display element.

In the following description, an example of using a light-emittingelement that emits white light as a display element will be described;however, the display element is not limited to such an element. Forexample, light-emitting elements that emit light of different colors maybe included so that the light of different colors can be emitted fromadjacent pixels.

For the substrate 2510 and the substrate 2570, for example, a flexiblematerial with a vapor permeability of lower than or equal to 1×10⁻⁵g·m⁻²·day⁻¹, preferably lower than or equal to 1×10⁻⁶ g·m⁻²·day⁻¹ can befavorably used. Alternatively, materials whose thermal expansioncoefficients are substantially equal to each other are preferably usedfor the substrate and the substrate 2570. For example, the coefficientsof linear expansion of the materials are preferably lower than or equalto 1×10⁻³/K, further preferably lower than or equal to 5×10⁻⁵/K, andstill further preferably lower than or equal to 1×10⁻⁵/K.

Note that the substrate 2510 is a stacked body including an insulatinglayer 2510 a for preventing impurity diffusion into the light-emittingelement, a flexible substrate 2510 b, and an adhesive layer 2510 c forattaching the insulating layer 2510 a and the flexible substrate 2510 bto each other. The substrate 2570 is a stacked body including aninsulating layer 2570 a for preventing impurity diffusion into thelight-emitting element, a flexible substrate 2570 b, and an adhesivelayer 2570 c for attaching the insulating layer 2570 a and the flexiblesubstrate 2570 b to each other.

For the adhesive layer 2510 c and the adhesive layer 2570 c, forexample, polyester, polyolefin, polyamide (e.g., nylon, aramid),polyimide, polycarbonate, or an acrylic resin, polyurethane, or an epoxyresin can be used. Alternatively, a material that includes a resinhaving a siloxane bond such as silicone can be used.

A sealing layer 2560 is provided between the substrate 2510 and thesubstrate 2570. The sealing layer 2560 preferably has a refractive indexhigher than that of air. In the case where light is extracted to thesealing layer 2560 side as illustrated in FIG. 24A, the sealing layercan also serve as an optical adhesive layer.

A sealant may be formed in the peripheral portion of the sealing layer2560. With the use of the sealant, a light-emitting element 2550R can beprovided in a region surrounded by the substrate 2510, the substrate2570, the sealing layer 2560, and the sealant. Note that an inert gas(such as nitrogen and argon) may be used instead of the sealing layer2560. A drying agent may be provided in the inert gas so as to adsorbmoisture or the like. A resin such as an acrylic resin or an epoxy resinmay be used. An epoxy-based resin or a glass frit is preferably used asthe sealant. As a material used for the sealant, a material which isimpermeable to moisture and oxygen is preferably used.

The display device 2501 includes a pixel 2502R. The pixel 2502R includesa light-emitting module 2580R.

The pixel 2502R includes the light-emitting element 2550R and atransistor 2502 t that can supply electric power to the light-emittingelement 2550R. Note that the transistor 2502 t functions as part of thepixel circuit. The light-emitting module 2580R includes thelight-emitting element 2550R and a coloring layer 2567R.

The light-emitting element 2550R includes a lower electrode, an upperelectrode, and an EL layer between the lower electrode and the upperelectrode. As the light-emitting element 2550R, any of thelight-emitting elements described in Embodiments 1 to 3 can be used.

A microcavity structure may be employed between the lower electrode andthe upper electrode so as to increase the intensity of light having aspecific wavelength.

In the case where the sealing layer 2560 is provided on the lightextraction side, the sealing layer 2560 is in contact with thelight-emitting element 2550R and the coloring layer 2567R.

The coloring layer 2567R is positioned in a region overlapping with thelight-emitting element 2550R. Accordingly, part of light emitted fromthe light-emitting element 2550R passes through the coloring layer 2567Rand is emitted to the outside of the light-emitting module 2580R asindicated by an arrow in the drawing.

The display device 2501 includes a light-blocking layer 2567BM on thelight extraction side. The light-blocking layer 2567BM is provided so asto surround the coloring layer 2567R.

The coloring layer 2567R is a coloring layer having a function oftransmitting light in a particular wavelength region. For example, acolor filter for transmitting light in a red wavelength region, a colorfilter for transmitting light in a green wavelength region, a colorfilter for transmitting light in a blue wavelength region, a colorfilter for transmitting light in a yellow wavelength region, or the likecan be used. Each color filter can be formed with any of variousmaterials by a printing method, an inkjet method, an etching methodusing a photolithography technique, or the like.

An insulating layer 2521 is provided in the display device 2501. Theinsulating layer covers the transistor 2502 t. Note that the insulatinglayer 2521 has a function of covering unevenness caused by the pixelcircuit. The insulating layer 2521 may have a function of suppressingimpurity diffusion. This can prevent the reliability of the transistor2502 t or the like from being lowered by impurity diffusion.

The light-emitting element 2550R is formed over the insulating layer2521. A partition is provided so as to overlap with an end portion ofthe lower electrode of the light-emitting element 2550R. Note that aspacer for controlling the distance between the substrate 2510 and thesubstrate 2570 may be formed over the partition 2528.

A scan line driver circuit 2503 g(1) includes a transistor 2503 t and acapacitor 2503 c. Note that the driver circuit can be formed in the sameprocess and over the same substrate as those of the pixel circuits.

The wirings 2511 through which signals can be supplied are provided overthe substrate 2510. The terminal 2519 is provided over the wirings 2511.The FPC 2509(1) is electrically connected to the terminal 2519. The FPC2509(1) has a function of supplying a video signal, a clock signal, astart signal, a reset signal, or the like. Note that the FPC 2509(1) maybe provided with a PWB.

In the display device 2501, transistors with any of a variety ofstructures can be used. FIG. 24A illustrates an example of usingbottom-gate transistors; however, the present invention is not limitedto this example, and top-gate transistors may be used in the displaydevice 2501 as illustrated in FIG. 24B.

In addition, there is no particular limitation on the polarity of thetransistor 2502 t and the transistor 2503 t. For these transistors,n-channel and p-channel transistors may be used, or either n-channeltransistors or p-channel transistors may be used, for example.Furthermore, there is no particular limitation on the crystallinity of asemiconductor film used for the transistors 2502 t and 2503 t. Forexample, an amorphous semiconductor film or a crystalline semiconductorfilm may be used. Examples of semiconductor materials include Group 14semiconductors (e.g., a semiconductor including silicon), compoundsemiconductors (including oxide semiconductors), organic semiconductors,and the like. An oxide semiconductor that has an energy gap of 2 eV ormore, preferably 2.5 eV or more, further preferably 3 eV or more ispreferably used for one of the transistors 2502 t and 2503 t or both, sothat the off-state current of the transistors can be reduced. Examplesof the oxide semiconductors include an In—Ga oxide, an In-M-Zn oxide (Mrepresents Al, Ga, Y, Zr, La, Ce, Sn, Hf, or Nd), and the like.

<Description of Touch Sensor>

Next, the touch sensor 2595 will be described in detail with referenceto FIG. 24C. FIG. 24C corresponds to a cross-sectional view taken alongdashed-dotted line X3-X4 in FIG. 23B.

The touch sensor 2595 includes the electrodes 2591 and the electrodes2592 provided in a staggered arrangement on the substrate 2590, aninsulating layer 2593 covering the electrodes and the electrodes 2592,and the wiring 2594 that electrically connects the adjacent electrodes2591 to each other.

The electrodes 2591 and the electrodes 2592 are formed using alight-transmitting conductive material. As a light-transmittingconductive material, a conductive oxide such as indium oxide, indium tinoxide, indium zinc oxide, zinc oxide, or zinc oxide to which gallium isadded can be used. Note that a film including graphene may be used aswell. The film including graphene can be formed, for example, byreducing a film containing graphene oxide. As a reducing method, amethod with application of heat or the like can be employed.

The electrodes 2591 and the electrodes 2592 may be formed by, forexample, depositing a light-transmitting conductive material on thesubstrate 2590 by a sputtering method and then removing an unnecessaryportion by any of various pattern forming techniques such asphotolithography.

Examples of a material for the insulating layer 2593 are a resin such asan acrylic resin or an epoxy resin, a resin having a siloxane bond suchas silicone, and an inorganic insulating material such as silicon oxide,silicon oxynitride, or aluminum oxide.

Openings reaching the electrodes 2591 are formed in the insulating layer2593, and the wiring 2594 electrically connects the adjacent electrodes2591. Alight-transmitting conductive material can be favorably used asthe wiring 2594 because the aperture ratio of the touch panel can beincreased. Moreover, a material with higher conductivity than theconductivities of the electrodes 2591 and 2592 can be favorably used forthe wiring 2594 because electric resistance can be reduced.

One electrode 2592 extends in one direction, and a plurality ofelectrodes 2592 are provided in the form of stripes. The wiring 2594intersects with the electrode 2592.

Adjacent electrodes 2591 are provided with one electrode 2592 providedtherebetween. The wiring 2594 electrically connects the adjacentelectrodes 2591.

Note that the plurality of electrodes 2591 are not necessarily arrangedin the direction orthogonal to one electrode 2592 and may be arranged tointersect with one electrode 2592 at an angle of more than 0 degrees andless than 90 degrees.

The wiring 2598 is electrically connected to any of the electrodes 2591and 2592. Part of the wiring 2598 functions as a terminal. For thewiring 2598, a metal material such as aluminum, gold, platinum, silver,nickel, titanium, tungsten, chromium, molybdenum, iron, cobalt, copper,or palladium or an alloy material containing any of these metalmaterials can be used.

Note that an insulating layer that covers the insulating layer 2593 andthe wiring 2594 may be provided to protect the touch sensor 2595.

A connection layer 2599 electrically connects the wiring 2598 to the FPC2509(2).

As the connection layer 2599, any of various anisotropic conductivefilms (ACF), anisotropic conductive pastes (ACP), or the like can beused.

<Description 2 of Touch Panel>

Next, the touch panel 2000 will be described in detail with reference toFIG. 25A. FIG. 25A corresponds to a cross-sectional view taken alongdashed-dotted line X5-X6 in FIG. 23A.

In the touch panel 2000 illustrated in FIG. 25A, the display device 2501described with reference to FIG. 24A and the touch sensor 2595 describedwith reference to FIG. 24C are attached to each other.

The touch panel 2000 illustrated in FIG. 25A includes an adhesive layer2597 and an anti-reflective layer 2567 p in addition to the componentsdescribed with reference to FIGS. 24A and 24C.

The adhesive layer 2597 is provided in contact with the wiring 2594.Note that the adhesive layer 2597 attaches the substrate 2590 to thesubstrate 2570 so that the touch sensor 2595 overlaps with the displaydevice 2501. The adhesive layer 2597 preferably has a light-transmittingproperty. A heat curable resin or an ultraviolet curable resin can beused for the adhesive layer 2597. For example, an acrylic resin, aurethane-based resin, an epoxy-based resin, or a siloxane-based resincan be used.

The anti-reflective layer 2567 p is positioned in a region overlappingwith pixels. As the anti-reflective layer 2567 p, a circularlypolarizing plate can be used, for example.

Next, a touch panel having a structure different from that illustratedin FIG. 25A will be described with reference to FIG. 25B.

FIG. 25B is a cross-sectional view of a touch panel 2001. The touchpanel 2001 illustrated in FIG. 25B differs from the touch panel 2000illustrated in FIG. 25A in the position of the touch sensor 2595relative to the display device 2501. Different parts are described indetail below, and the above description of the touch panel 2000 isreferred to for the other similar parts.

The coloring layer 2567R is positioned in a region overlapping with thelight-emitting element 2550R. The light-emitting element 2550Rillustrated in FIG. 25B emits light to the side where the transistor2502 t is provided. Accordingly, part of light emitted from thelight-emitting element 2550R passes through the coloring layer 2567R andis emitted to the outside of the light-emitting module 2580R asindicated by an arrow in FIG. 25B.

The touch sensor 2595 is provided on the substrate 2510 side of thedisplay device 2501.

The adhesive layer 2597 is provided between the substrate 2510 and thesubstrate 2590 and attaches the touch sensor 2595 to the display device2501.

As illustrated in FIG. 25A or 25B, light may be emitted from thelight-emitting element through one or both of the substrate 2510 and thesubstrate 2570.

<Description of Method for Driving Touch Panel>

Next, an example of a method for driving a touch panel will be describedwith reference to FIGS. 26A and 26B.

FIG. 26A is a block diagram illustrating the structure of a mutualcapacitive touch sensor. FIG. 26A illustrates a pulse voltage outputcircuit 2601 and a current sensing circuit 2602. Note that in FIG. 26A,six wirings X1 to X6 represent the electrodes 2621 to which a pulsevoltage is applied, and six wirings Y1 to Y6 represent the electrodes2622 that detect changes in current. FIG. 26A also illustratescapacitors 2603 that are each formed in a region where the electrodes2621 and 2622 overlap with each other. Note that functional replacementbetween the electrodes 2621 and 2622 is possible.

The pulse voltage output circuit 2601 is a circuit for sequentiallyapplying a pulse voltage to the wirings X1 to X6. By application of apulse voltage to the wirings X1 to X6, an electric field is generatedbetween the electrodes 2621 and 2622 of the capacitor 2603. When theelectric field between the electrodes is shielded, for example, a changeoccurs in the capacitor 2603 (mutual capacitance). The approach orcontact of a sensing target can be sensed by utilizing this change.

The current sensing circuit 2602 is a circuit for detecting changes incurrent flowing through the wirings Y1 to Y6 that are caused by thechange in mutual capacitance in the capacitor 2603. No change in currentvalue is detected in the wirings Y1 to Y6 when there is no approach orcontact of a sensing target, whereas a decrease in current value isdetected when mutual capacitance is decreased owing to the approach orcontact of a sensing target. Note that an integrator circuit or the likeis used for sensing of current values.

FIG. 26B is a timing chart showing input and output waveforms in themutual capacitive touch sensor illustrated in FIG. 26A. In FIG. 26B,sensing of a sensing target is performed in all the rows and columns inone frame period. FIG. 26B shows a period when a sensing target is notsensed (not touched) and a period when a sensing target is sensed(touched). In FIG. 26B, sensed current values of the wirings Y1 to Y6are shown as the waveforms of voltage values.

A pulse voltage is sequentially applied to the wirings X1 to X6, and thewaveforms of the wirings Y1 to Y6 change in accordance with the pulsevoltage. When there is no approach or contact of a sensing target, thewaveforms of the wirings Y1 to Y6 change uniformly in accordance withchanges in the voltages of the wirings X1 to X6. The current value isdecreased at the point of approach or contact of a sensing target andaccordingly the waveform of the voltage value changes.

By detecting a change in mutual capacitance in this manner, the approachor contact of a sensing target can be sensed.

<Description of Sensor Circuit>

Although FIG. 26A illustrates a passive matrix type touch sensor inwhich only the capacitor 2603 is provided at the intersection of wiringsas a touch sensor, an active matrix type touch sensor including atransistor and a capacitor may be used. FIG. 27 illustrates an exampleof a sensor circuit included in an active matrix type touch sensor.

The sensor circuit in FIG. 27 includes the capacitor 2603 andtransistors 2611, 2612, and 2613.

A signal G2 is input to a gate of the transistor 2613. A voltage VRES isapplied to one of a source and a drain of the transistor 2613, and oneelectrode of the capacitor 2603 and a gate of the transistor 2611 areelectrically connected to the other of the source and the drain of thetransistor 2613. One of a source and a drain of the transistor 2611 iselectrically connected to one of a source and a drain of the transistor2612, and a voltage VSS is applied to the other of the source and thedrain of the transistor 2611. A signal G1 is input to agate of thetransistor 2612, and a wiring ML is electrically connected to the otherof the source and the drain of the transistor 2612. The voltage VSS isapplied to the other electrode of the capacitor 2603.

Next, the operation of the sensor circuit in FIG. 27 will be described.First, a potential for turning on the transistor 2613 is supplied as thesignal G2, and a potential with respect to the voltage VRES is thusapplied to the node n connected to the gate of the transistor 2611.Then, a potential for turning off the transistor 2613 is applied as thesignal G2, whereby the potential of the node n is maintained.

Then, mutual capacitance of the capacitor 2603 changes owing to theapproach or contact of a sensing target such as a finger, andaccordingly the potential of the node n is changed from VRES.

In reading operation, a potential for turning on the transistor 2612 issupplied as the signal G1. A current flowing through the transistor2611, that is, a current flowing through the wiring ML is changed inaccordance with the potential of the node n. By sensing this current,the approach or contact of a sensing target can be sensed.

In each of the transistors 2611, 2612, and 2613, an oxide semiconductorlayer is preferably used as a semiconductor layer in which a channelregion is formed. In particular, such a transistor is preferably used asthe transistor 2613 so that the potential of the node n can be held fora long time and the frequency of operation of resupplying VRES to thenode n (refresh operation) can be reduced.

The structure described in this embodiment can be used in combinationwith any of the structures described in the other embodiments asappropriate.

Embodiment 7

In this embodiment, a display module and electronic devices including alight-emitting element of one embodiment of the present invention willbe described with reference to FIG. 28, FIGS. 29A to 29G, FIGS. 30A to30F, FIGS. 31A to 31D, and FIGS. 32A and 32B.

<Display Module>

In a display module 8000 in FIG. 28, a touch sensor 8004 connected to anFPC 8003, a display device 8006 connected to an FPC 8005, a frame 8009,a printed board 8010, and a battery 8011 are provided between an uppercover 8001 and a lower cover 8002.

The light-emitting element of one embodiment of the present inventioncan be used for the display device 8006, for example.

The shapes and sizes of the upper cover 8001 and the lower cover 8002can be changed as appropriate in accordance with the sizes of the touchsensor 8004 and the display device 8006.

The touch sensor 8004 can be a resistive touch sensor or a capacitivetouch sensor and may be formed to overlap with the display device 8006.A counter substrate (sealing substrate) of the display device 8006 canhave a touch sensor function. A photosensor may be provided in eachpixel of the display device 8006 so that an optical touch sensor isobtained.

The frame 8009 protects the display device 8006 and also serves as anelectromagnetic shield for blocking electromagnetic waves generated bythe operation of the printed board 8010. The frame 8009 may serve as aradiator plate.

The printed board 8010 has a power supply circuit and a signalprocessing circuit for outputting a video signal and a clock signal. Asa power source for supplying power to the power supply circuit, anexternal commercial power source or the battery 8011 provided separatelymay be used. The battery 8011 can be omitted in the case of using acommercial power source.

The display module 8000 can be additionally provided with a member suchas a polarizing plate, a retardation plate, or a prism sheet.

<Electronic Devices>

FIGS. 29A to 29G illustrate electronic devices. These electronic devicescan include a housing 9000, a display portion 9001, a speaker 9003,operation keys 9005 (including a power switch or an operation switch), aconnection terminal 9006, a sensor 9007 (a sensor having a function ofmeasuring or sensing force, displacement, position, speed, acceleration,angular velocity, rotational frequency, distance, light, liquid,magnetism, temperature, chemical substance, sound, time, hardness,electric field, current, voltage, electric power, radiation, flow rate,humidity, gradient, oscillation, odor, or infrared ray), a microphone9008, and the like. In addition, the sensor 9007 may have a function ofmeasuring biological information like a pulse sensor and a finger printsensor.

The electronic devices illustrated in FIGS. 29A to 29G can have avariety of functions, for example, a function of displaying a variety ofdata (a still image, a moving image, a text image, and the like) on thedisplay portion, a touch sensor function, a function of displaying acalendar, date, time, and the like, a function of controlling a processwith a variety of software (programs), a wireless communicationfunction, a function of being connected to a variety of computernetworks with a wireless communication function, a function oftransmitting and receiving a variety of data with a wirelesscommunication function, a function of reading a program or data storedin a memory medium and displaying the program or data on the displayportion, and the like. Note that functions that can be provided for theelectronic devices illustrated in FIGS. 29A to 29G are not limited tothose described above, and the electronic devices can have a variety offunctions. Although not illustrated in FIGS. 29A to 29G, the electronicdevices may include a plurality of display portions. The electronicdevices may have a camera or the like and a function of taking a stillimage, a function of taking a moving image, a function of storing thetaken image in a memory medium (an external memory medium or a memorymedium incorporated in the camera), a function of displaying the takenimage on the display portion, or the like.

The electronic devices illustrated in FIGS. 29A to 29G will be describedin detail below.

FIG. 29A is a perspective view of a portable information terminal 9100.The display portion 9001 of the portable information terminal 9100 isflexible. Therefore, the display portion 9001 can be incorporated alonga bent surface of a bent housing 9000. In addition, the display portion9001 includes a touch sensor, and operation can be performed by touchingthe screen with a finger, a stylus, or the like. For example, when anicon displayed on the display portion 9001 is touched, an applicationcan be started.

FIG. 29B is a perspective view of a portable information terminal 9101.The portable information terminal 9101 functions as, for example, one ormore of a telephone set, a notebook, and an information browsing system.Specifically, the portable information terminal can be used as asmartphone. Note that the speaker 9003, the connection terminal 9006,the sensor 9007, and the like, which are not shown in FIG. 29B, can bepositioned in the portable information terminal 9101 as in the portableinformation terminal 9100 shown in FIG. 29A. The portable informationterminal 9101 can display characters and image information on itsplurality of surfaces. For example, three operation buttons 9050 (alsoreferred to as operation icons, or simply, icons) can be displayed onone surface of the display portion 9001. Furthermore, information 9051indicated by dashed rectangles can be displayed on another surface ofthe display portion 9001. Examples of the information 9051 includedisplay indicating reception of an incoming email, social networkingservice (SNS) message, call, and the like; the title and sender of anemail and SNS message; the date; the time; remaining battery; anddisplay indicating the strength of a received signal such as a radiowave. Instead of the information 9051, the operation buttons 9050 or thelike may be displayed on the position where the information 9051 isdisplayed.

As a material of the housing 9000, an alloy, plastic, ceramic, or amaterial containing carbon fiber can be used. As the material containingcarbon fiber, carbon fiber reinforced plastic (CFRP) has advantages oflightweight and corrosion-free; however, it is black and thus limits theexterior and design of the housing. The CFRP can be regarded as a kindof reinforced plastic, which may use glass fiber or aramid fiber. Sincethe fiber might be separated from a resin by high impact, the alloy ispreferred. As the alloy, an aluminum alloy and a magnesium alloy can begiven. An amorphous alloy (also referred to as metallic glass)containing zirconium, copper, nickel, and titanium especially has highelastic strength. This amorphous alloy has a glass transition region atroom temperature, which is also referred to as a bulk-solidifyingamorphous alloy and substantially has an amorphous atomic structure. Analloy material is molded in a mold of at least the part of the housingand coagulated by a solidification casting method, whereby part of thehousing is formed with the bulk-solidifying amorphous alloy. Theamorphous alloy may contain beryllium, silicon, niobium, boron, gallium,molybdenum, tungsten, manganese, iron, cobalt, yttrium, vanadium,phosphorus, carbon, or the like in addition to zirconium, copper,nickel, and titanium. The amorphous alloy may be formed by a vacuumevaporation method, a sputtering method, an electroplating method, anelectroless plating method, or the like instead of the solidificationcasting method. The amorphous alloy may include a microcrystal or ananocrystal as long as a state without a long-range order (a periodicstructure) is maintained as a whole. Note that the term alloy includesboth a complete solid solution alloy having a single solid-phasestructure and a partial solution having two or more phases. The housing9000 using the amorphous alloy can have high elastic strength. Even ifthe portable information terminal 9101 is dropped and the impact causestemporary deformation, the use of the amorphous alloy in the housing9000 allows a return to the original shape; thus, the impact resistanceof the portable information terminal 9101 can be improved.

FIG. 29C is a perspective view of a portable information terminal 9102.The portable information terminal 9102 has a function of displayinginformation on three or more surfaces of the display portion 9001. Here,information 9052, information 9053, and information 9054 are displayedon different surfaces. For example, a user of the portable informationterminal 9102 can see the display (here, the information 9053) with theportable information terminal 9102 put in a breast pocket of his/herclothes. Specifically, a caller's phone number, name, or the like of anincoming call is displayed in a position that can be seen from above theportable information terminal 9102. Thus, the user can see the displaywithout taking out the portable information terminal 9102 from thepocket and decide whether to answer the call.

FIG. 29D is a perspective view of a watch-type portable informationterminal 9200. The portable information terminal 9200 is capable ofexecuting a variety of applications such as mobile phone calls,e-mailing, viewing and editing texts, music reproduction, Internetcommunication, and computer games. The display surface of the displayportion 9001 is bent, and images can be displayed on the bent displaysurface. The portable information terminal can employ near fieldcommunication that is a communication method based on an existingcommunication standard. In that case, for example, mutual communicationbetween the portable information terminal 9200 and a headset capable ofwireless communication can be performed, and thus hands-free calling ispossible. The portable information terminal 9200 includes the connectionterminal 9006, and data can be directly transmitted to and received fromanother information terminal via a connector. Power charging through theconnection terminal is possible. Note that the charging operation may beperformed by wireless power feeding without using the connectionterminal 9006.

FIGS. 29E, 29F, and 29G are perspective views of a foldable portableinformation terminal 9201. FIG. 29E is a perspective view illustratingthe portable information terminal 9201 that is opened. FIG. 29F is aperspective view illustrating the portable information terminal 9201that is being opened or being folded. FIG. 29G is a perspective viewillustrating the portable information terminal 9201 that is folded. Theportable information terminal 9201 is highly portable when folded. Whenthe portable information terminal 9201 is opened, a seamless largedisplay region is highly browsable. The display portion 9001 of theportable information terminal 9201 is supported by three housings 9000joined together by hinges 9055. By folding the portable informationterminal 9201 at a connection portion between two housings 9000 with thehinges 9055, the portable information terminal 9201 can be reversiblychanged in shape from an opened state to a folded state. For example,the portable information terminal 9201 can be bent with a radius ofcurvature of greater than or equal to 1 mm and less than or equal to 150mm.

Examples of electronic devices are a television set (also referred to asa television or a television receiver), a monitor of a computer or thelike, a camera such as a digital camera or a digital video camera, adigital photo frame, a mobile phone handset (also referred to as amobile phone or a mobile phone device), a goggle-type display (headmounted display), a portable game machine, a portable informationterminal, an audio reproducing device, and a large-sized game machinesuch as a pachinko machine.

Furthermore, the electronic device of one embodiment of the presentinvention may include a secondary battery. It is preferable that thesecondary battery be capable of being charged by non-contact powertransmission.

Examples of the secondary battery include a lithium ion secondarybattery such as a lithium polymer battery using a gel electrolyte(lithium ion polymer battery), a lithium-ion battery, a nickel-hydridebattery, a nickel-cadmium battery, an organic radical battery, a leadstorage battery, an air secondary battery, a nickel-zinc battery, and asilver-zinc battery.

The electronic device of one embodiment of the present invention mayinclude an antenna. When a signal is received by the antenna, theelectronic device can display an image, data, or the like on a displayportion. When the electronic device includes a secondary battery, theantenna may be used for non-contact power transmission.

FIG. 30A illustrates a portable game machine including a housing 7101, ahousing 7102, display portions 7103 and 7104, a microphone 7105,speakers 7106, an operation key 7107, a stylus 7108, and the like. Whenthe light-emitting device of one embodiment of the present invention isused as the display portion 7103 or 7104, it is possible to provide auser-friendly portable game machine with quality that hardlydeteriorates. Although the portable game machine illustrated in FIG. 30Aincludes two display portions, the display portions 7103 and 7104, thenumber of display portions included in the portable game machine is notlimited to two.

FIG. 30B illustrates a video camera including a housing 7701, a housing7702, a display portion 7703, operation keys 7704, a lens 7705, a joint7706, and the like. The operation keys 7704 and the lens 7705 areprovided for the housing 7701, and the display portion 7703 is providedfor the housing 7702. The housing 7701 and the housing 7702 areconnected to each other with the joint 7706, and the angle between thehousing 7701 and the housing 7702 can be changed with the joint 7706.Images displayed on the display portion 7703 may be switched inaccordance with the angle at the joint 7706 between the housing 7701 andthe housing 7702.

FIG. 30C illustrates a notebook personal computer including a housing7121, a display portion 7122, a keyboard 7123, a pointing device 7124,and the like. Note that the display portion 7122 is small- ormedium-sized but can perform 8 k display because it has greatly highpixel density and high resolution; therefore, a significantly clearimage can be obtained.

FIG. 30D is an external view of a head-mounted display 7200.

The head-mounted display 7200 includes a mounting portion 7201, a lens7202, a main body 7203, a display portion 7204, a cable 7205, and thelike. The mounting portion 7201 includes a battery 7206.

Power is supplied from the battery 7206 to the main body 7203 throughthe cable 7205. The main body 7203 includes a wireless receiver or thelike to receive video data, such as image data, and display it on thedisplay portion 7204. The movement of the eyeball and the eyelid of auser is captured by a camera in the main body 7203 and then coordinatesof the points the user looks at are calculated using the captured datato utilize the eye point of the user as an input means.

The mounting portion 7201 may include a plurality of electrodes so as tobe in contact with the user. The main body 7203 may be configured tosense current flowing through the electrodes with the movement of theuser's eyeball to recognize the direction of his or her eyes. The mainbody 7203 may be configured to sense current flowing through theelectrodes to monitor the user's pulse. The mounting portion 7201 mayinclude sensors, such as a temperature sensor, a pressure sensor, or anacceleration sensor, so that the user's biological information can bedisplayed on the display portion 7204. The main body 7203 may beconfigured to sense the movement of the user's head or the like to movean image displayed on the display portion 7204 in synchronization withthe movement of the user's head or the like.

FIG. 30E is an external view of a camera 7300. The camera 7300 includesa housing 7301, a display portion 7302, an operation button 7303, ashutter button 7304, a connection portion 7305, and the like. A lens7306 can be put on the camera 7300.

The connection portion 7305 includes an electrode to connect with afinder 7400, which is described below, a stroboscope, or the like.

Although the lens 7306 of the camera 7300 here is detachable from thehousing 7301 for replacement, the lens 7306 may be included in thehousing 7301.

Images can be taken at the touch of the shutter button 7304. Inaddition, images can be taken by operation of the display portion 7302including a touch sensor.

In the display portion 7302, the display device of one embodiment of thepresent invention or a touch sensor can be used.

FIG. 30F shows the camera 7300 with the finder 7400 connected.

The finder 7400 includes a housing 7401, a display portion 7402, and abutton 7403.

The housing 7401 includes a connection portion for engagement with theconnection portion 7305 of the camera 7300 so that the finder 7400 canbe connected to the camera 7300. The connection portion includes anelectrode, and an image or the like received from the camera through theelectrode can be displayed on the display portion 7402.

The button 7403 has a function of a power button, and the displayportion 7402 can be turned on and off with the button 7403.

Although the camera 7300 and the finder 7400 are separate and detachableelectronic devices in FIGS. 30E and 30F, the housing 7301 of the camera7300 may include a finder having a display device of one embodiment ofthe present invention or a touch sensor.

FIG. 31A illustrates an example of a television set. In the televisionset 9300, the display portion 9001 is incorporated into the housing9000. Here, the housing 9000 is supported by a stand 9301.

The television set 9300 illustrated in FIG. 31A can be operated with anoperation switch of the housing 9000 or a separate remote controller9311. The display portion 9001 may include a touch sensor. Thetelevision set 9300 can be operated by touching the display portion witha finger or the like. The remote controller 9311 may be provided with adisplay portion for displaying data output from the remote controller9311. With operation keys or a touch panel of the remote controller9311, channels or volume can be controlled and images displayed on thedisplay portion 9001 can be controlled.

The television set 9300 is provided with a receiver, a modem, or thelike. A general television broadcast can be received with the receiver.When the television set is connected to a communication network with orwithout wires via the modem, one-way (from a transmitter to a receiver)or two-way (between a transmitter and a receiver or between receivers)data communication can be performed.

The electronic device or the lighting device of one embodiment of thepresent invention has flexibility and therefore can be incorporatedalong a curved inside/outside wall surface of a house or a building or acurved interior/exterior surface of a car.

FIG. 31B is an external view of an automobile 9700. FIG. 31C illustratesa driver's seat of the automobile 9700. The automobile 9700 includes acar body 9701, wheels 9702, a dashboard 9703, lights 9704, and the like.The display device, the light-emitting device, or the like of oneembodiment of the present invention can be used in a display portion orthe like of the automobile 9700. For example, the display device, thelight-emitting device, or the like of one embodiment of the presentinvention can be used in display portions 9710 to 9715 illustrated inFIG. 31C.

The display portion 9710 and the display portion 9711 are each a displaydevice provided in an automobile windshield. The display device, thelight-emitting device, or the like of one embodiment of the presentinvention can be a see-through display device, through which theopposite side can be seen, using a light-transmitting conductivematerial for its electrodes and wirings. Such a see-through displayportion 9710 or 9711 does not hinder driver's vision during driving theautomobile 9700. Thus, the display device, the light-emitting device, orthe like of one embodiment of the present invention can be provided inthe windshield of the automobile 9700. Note that in the case where atransistor or the like for driving the display device, thelight-emitting device, or the like is provided, a transistor having alight-transmitting property, such as an organic transistor using anorganic semiconductor material or a transistor using an oxidesemiconductor, is preferably used.

The display portion 9712 is a display device provided on a pillarportion. For example, an image taken by an imaging unit provided in thecar body is displayed on the display portion 9712, whereby the viewhindered by the pillar portion can be compensated. The display portion9713 is a display device provided on the dashboard. For example, animage taken by an imaging unit provided in the car body is displayed onthe display portion 9713, whereby the view hindered by the dashboard canbe compensated. That is, by displaying an image taken by an imaging unitprovided on the outside of the automobile, blind areas can be eliminatedand safety can be increased. Displaying an image to compensate for thearea which a driver cannot see, makes it possible for the driver toconfirm safety easily and comfortably.

FIG. 31D illustrates the inside of a car in which bench seats are usedfor a driver seat and a front passenger seat. A display portion 9721 isa display device provided in a door portion. For example, an image takenby an imaging unit provided in the car body is displayed on the displayportion 9721, whereby the view hindered by the door can be compensated.A display portion 9722 is a display device provided in a steering wheel.A display portion 9723 is a display device provided in the middle of aseating face of the bench seat. Note that the display device can be usedas a seat heater by providing the display device on the seating face orbackrest and by using heat generation of the display device as a heatsource.

The display portion 9714, the display portion 9715, and the displayportion 9722 can provide a variety of kinds of information such asnavigation data, a speedometer, a tachometer, a mileage, a fuel meter, agearshift indicator, and air-condition setting. The content, layout, orthe like of the display on the display portions can be changed freely bya user as appropriate. The information listed above can also bedisplayed on the display portions 9710 to 9713, 9721, and 9723. Thedisplay portions 9710 to 9715 and 9721 to 9723 can also be used aslighting devices. The display portions 9710 to 9715 and 9721 to 9723 canalso be used as heating devices.

A display device 9500 illustrated in FIGS. 32A and 32B includes aplurality of display panels 9501, a hinge 9511, and a bearing 9512. Theplurality of display panels 9501 each include a display region 9502 anda light-transmitting region 9503.

Each of the plurality of display panels 9501 is flexible. Two adjacentdisplay panels 9501 are provided so as to partly overlap with eachother. For example, the light-transmitting regions 9503 of the twoadjacent display panels 9501 can be overlapped each other. A displaydevice having a large screen can be obtained with the plurality ofdisplay panels 9501. The display device is highly versatile because thedisplay panels 9501 can be wound depending on its use.

Moreover, although the display regions 9502 of the adjacent displaypanels 9501 are separated from each other in FIGS. 32A and 32B, withoutlimitation to this structure, the display regions 9502 of the adjacentdisplay panels 9501 may overlap with each other without any space sothat a continuous display region 9502 is obtained, for example.

The electronic devices described in this embodiment each include thedisplay portion for displaying some sort of data. Note that thelight-emitting element of one embodiment of the present invention canalso be used for an electronic device which does not have a displayportion. The structure in which the display portion of the electronicdevice described in this embodiment is flexible and display can beperformed on the bent display surface or the structure in which thedisplay portion of the electronic device is foldable is described as anexample; however, the structure is not limited thereto and a structurein which the display portion of the electronic device is not flexibleand display is performed on a plane portion may be employed.

The structure described in this embodiment can be used in combinationwith any of the structures described in the other embodiments asappropriate.

Embodiment 8

In this embodiment, a light-emitting device including the light-emittingelement of one embodiment of the present invention will be describedwith reference to FIGS. 33A to 33C and FIGS. 34A to 34D.

FIG. 33A is a perspective view of a light-emitting device 3000 shown inthis embodiment, and FIG. 33B is a cross-sectional view alongdashed-dotted line E-F in FIG. 33A. Note that in FIG. 33A, somecomponents are illustrated by broken lines in order to avoid complexityof the drawing.

The light-emitting device 3000 illustrated in FIGS. 33A and 33B includesa substrate 3001, a light-emitting element 3005 over the substrate 3001,a first sealing region 3007 provided around the light-emitting element3005, and a second sealing region 3009 provided around the first sealingregion 3007.

Light is emitted from the light-emitting element 3005 through one orboth of the substrate 3001 and a substrate 3003. In FIGS. 33A and 33B, astructure in which light is emitted from the light-emitting element 3005to the lower side (the substrate 3001 side) is illustrated.

As illustrated in FIGS. 33A and 33B, the light-emitting device 3000 hasa double sealing structure in which the light-emitting element 3005 issurrounded by the first sealing region 3007 and the second sealingregion 3009. With the double sealing structure, entry of impurities(e.g., water, oxygen, and the like) from the outside into thelight-emitting element 3005 can be favorably suppressed. Note that it isnot necessary to provide both the first sealing region 3007 and thesecond sealing region 3009. For example, only the first sealing region3007 may be provided.

Note that in FIG. 33B, the first sealing region 3007 and the secondsealing region 3009 are each provided in contact with the substrate 3001and the substrate 3003. However, without limitation to such a structure,for example, one or both of the first sealing region 3007 and the secondsealing region 3009 may be provided in contact with an insulating filmor a conductive film provided on the substrate 3001. Alternatively, oneor both of the first sealing region 3007 and the second sealing region3009 may be provided in contact with an insulating film or a conductivefilm provided on the substrate 3003.

The substrate 3001 and the substrate 3003 can have structures similar tothose of the substrate 200 and the substrate 220 described in the aboveembodiment, respectively. The light-emitting element 3005 can have astructure similar to that of any of the light-emitting elementsdescribed in the above embodiments.

For the first sealing region 3007, a material containing glass (e.g., aglass frit, a glass ribbon, and the like) can be used. For the secondsealing region 3009, a material containing a resin can be used. With theuse of the material containing glass for the first sealing region 3007,productivity and a sealing property can be improved. Moreover, with theuse of the material containing a resin for the second sealing region3009, impact resistance and heat resistance can be improved. However,the materials used for the first sealing region 3007 and the secondsealing region 3009 are not limited to such, and the first sealingregion 3007 may be formed using the material containing a resin and thesecond sealing region 3009 may be formed using the material containingglass.

The glass frit may contain, for example, magnesium oxide, calcium oxide,strontium oxide, barium oxide, cesium oxide, sodium oxide, potassiumoxide, boron oxide, vanadium oxide, zinc oxide, tellurium oxide,aluminum oxide, silicon dioxide, lead oxide, tin oxide, phosphorusoxide, ruthenium oxide, rhodium oxide, iron oxide, copper oxide,manganese dioxide, molybdenum oxide, niobium oxide, titanium oxide,tungsten oxide, bismuth oxide, zirconium oxide, lithium oxide, antimonyoxide, lead borate glass, tin phosphate glass, vanadate glass, orborosilicate glass. The glass frit preferably contains at least one kindof transition metal to absorb infrared light.

As the above glass frits, for example, a frit paste is applied to asubstrate and is subjected to heat treatment, laser light irradiation,or the like. The frit paste contains the glass frit and a resin (alsoreferred to as a binder) diluted by an organic solvent. Note that anabsorber which absorbs light having the wavelength of laser light may beadded to the glass frit. For example, an Nd:YAG laser or a semiconductorlaser is preferably used as the laser. The shape of laser light may becircular or quadrangular.

As the above material containing a resin, for example, polyester,polyolefin, polyamide (e.g., nylon, aramid), polyimide, polycarbonate,or an acrylic resin, polyurethane, or an epoxy resin can be used.Alternatively, a material that includes a resin having a siloxane bondsuch as silicone can be used.

Note that in the case where the material containing glass is used forone or both of the first sealing region 3007 and the second sealingregion 3009, the material containing glass preferably has a thermalexpansion coefficient close to that of the substrate 3001. With theabove structure, generation of a crack in the material containing glassor the substrate 3001 due to thermal stress can be suppressed.

For example, the following advantageous effect can be obtained in thecase where the material containing glass is used for the first sealingregion 3007 and the material containing a resin is used for the secondsealing region 3009.

The second sealing region 3009 is provided closer to an outer portion ofthe light-emitting device 3000 than the first sealing region 3007 is. Inthe light-emitting device 3000, distortion due to external force or thelike increases toward the outer portion. Thus, the light-emitting device3000 is sealed using the material containing a resin for the outerportion of the light-emitting device 3000 where a larger amount ofdistortion is generated, that is, the second sealing region 3009, andthe light-emitting device 3000 is sealed using the material containingglass for the first sealing region 3007 provided on an inner side of thesecond sealing region 3009, whereby the light-emitting device 3000 isless likely to be damaged even when distortion due to external force orthe like is generated.

Furthermore, as illustrated in FIG. 33B, a first region 3011 correspondsto the region surrounded by the substrate 3001, the substrate 3003, thefirst sealing region 3007, and the second sealing region 3009. A secondregion 3013 corresponds to the region surrounded by the substrate 3001,the substrate 3003, the light-emitting element 3005, and the firstsealing region 3007.

The first region 3011 and the second region 3013 are preferably filledwith, for example, an inert gas such as a rare gas or a nitrogen gas.Alternatively, the first region 3011 and the second region 3013 arepreferably filled with a resin such as an acrylic resin or an epoxyresin. Note that for the first region 3011 and the second region 3013, areduced pressure state is preferred to an atmospheric pressure state.

FIG. 33C illustrates a modification example of the structure in FIG.33B. FIG. 33C is a cross-sectional view illustrating the modificationexample of the light-emitting device 3000.

FIG. 33C illustrates a structure in which a desiccant 3018 is providedin a recessed portion provided in part of the substrate 3003. The othercomponents are the same as those of the structure illustrated in FIG.33B.

As the desiccant 3018, a substance which adsorbs moisture and the likeby chemical adsorption or a substance which adsorbs moisture and thelike by physical adsorption can be used. Examples of the substance thatcan be used as the desiccant 3018 include alkali metal oxides, alkalineearth metal oxide (e.g., calcium oxide, barium oxide, and the like),sulfate, metal halides, perchlorate, zeolite, silica gel, and the like.

Next, modification examples of the light-emitting device 3000 which isillustrated in FIG. 33B are described with reference to FIGS. 34A to34D. Note that FIGS. 34A to 34D are cross-sectional views illustratingthe modification examples of the light-emitting device 3000 illustratedin FIG. 33B.

In each of the light-emitting devices illustrated in FIGS. 34A to 34D,the second sealing region 3009 is not provided but only the firstsealing region 3007 is provided. Moreover, in each of the light-emittingdevices illustrated in FIGS. 34A to 34D, a region 3014 is providedinstead of the second region 3013 illustrated in FIG. 33B.

For the region 3014, for example, polyester, polyolefin, polyamide(e.g., nylon, aramid), polyimide, polycarbonate, or an acrylic resin,polyurethane, or an epoxy resin can be used. Alternatively, a materialthat includes a resin having a siloxane bond such as silicone can beused.

When the above-described material is used for the region 3014, what iscalled a solid-sealing light-emitting device can be obtained.

In the light-emitting device illustrated in FIG. 34B, a substrate 3015is provided on the substrate 3001 side of the light-emitting deviceillustrated in FIG. 34A.

The substrate 3015 has unevenness as illustrated in FIG. 34B. With astructure in which the substrate 3015 having unevenness is provided onthe side through which light emitted from the light-emitting element3005 is extracted, the efficiency of extraction of light from thelight-emitting element 3005 can be improved. Note that instead of thestructure having unevenness and illustrated in FIG. 34B, a substratehaving a function as a diffusion plate may be provided.

In the light-emitting device illustrated in FIG. 34C, light is extractedthrough the substrate 3003 side, unlike in the light-emitting deviceillustrated in FIG. 34A, in which light is extracted through thesubstrate 3001 side.

The light-emitting device illustrated in FIG. 34C includes the substrate3015 on the substrate 3003 side. The other components are the same asthose of the light-emitting device illustrated in FIG. 34B.

In the light-emitting device illustrated in FIG. 34D, the substrate 3003and the substrate 3015 included in the light-emitting device illustratedin FIG. 34C are not provided but a substrate 3016 is provided.

The substrate 3016 includes first unevenness positioned closer to thelight-emitting element 3005 and second unevenness positioned fartherfrom the light-emitting element 3005. With the structure illustrated inFIG. 34D, the efficiency of extraction of light from the light-emittingelement 3005 can be further improved.

Thus, the use of the structure described in this embodiment can providea light-emitting device in which deterioration of a light-emittingelement due to impurities such as moisture and oxygen is suppressed.Alternatively, with the structure described in this embodiment, alight-emitting device having high light extraction efficiency can beobtained.

Note that the structure described in this embodiment can be combined asappropriate with any of the structures described in the otherembodiments.

Embodiment 9

In this embodiment, examples in which the light-emitting element of oneembodiment of the present invention is used for various lighting devicesand electronic devices will be described with reference to FIGS. 35A to35C and FIG. 36.

An electronic device or a lighting device that has a light-emittingregion with a curved surface can be obtained with the use of thelight-emitting element of one embodiment of the present invention whichis manufactured over a substrate having flexibility.

Furthermore, a light-emitting device to which one embodiment of thepresent invention is applied can also be used for lighting for motorvehicles, examples of which are lighting for a dashboard, a windshield,a ceiling, and the like.

FIG. 35A is a perspective view illustrating one surface of amultifunction terminal 3500, and FIG. 35B is a perspective viewillustrating the other surface of the multifunction terminal 3500. In ahousing 3502 of the multifunction terminal 3500, a display portion 3504,a camera 3506, lighting 3508, and the like are incorporated. Thelight-emitting device of one embodiment of the present invention can beused for the lighting 3508.

The lighting 3508 that includes the light-emitting device of oneembodiment of the present invention functions as a planar light source.Thus, unlike a point light source typified by an LED, the lighting 3508can provide light emission with low directivity. When the lighting 3508and the camera 3506 are used in combination, for example, imaging can beperformed by the camera 3506 with the lighting 3508 lighting orflashing. Because the lighting functions as a planar light source, aphotograph as if taken under natural light can be taken.

Note that the multifunction terminal 3500 illustrated in FIGS. 35A and35B can have a variety of functions as in the electronic devicesillustrated in FIGS. 29A to 29G.

The housing 3502 can include a speaker, a sensor (a sensor having afunction of measuring force, displacement, position, speed,acceleration, angular velocity, rotational frequency, distance, light,liquid, magnetism, temperature, chemical substance, sound, time,hardness, electric field, current, voltage, electric power, radiation,flow rate, humidity, gradient, oscillation, odor, or infrared rays), amicrophone, and the like. When a detection device including a sensor fordetecting inclination, such as a gyroscope or an acceleration sensor, isprovided inside the multifunction terminal 3500, display on the screenof the display portion 3504 can be automatically switched by determiningthe orientation of the multifunction terminal 3500 (whether themultifunction terminal is placed horizontally or vertically for alandscape mode or a portrait mode).

The display portion 3504 may function as an image sensor. For example,an image of a palm print, a fingerprint, or the like is taken when thedisplay portion 3504 is touched with the palm or the finger, wherebypersonal authentication can be performed. Furthermore, by providing abacklight or a sensing light source which emits near-infrared light inthe display portion 3504, an image of a finger vein, a palm vein, or thelike can be taken. Note that the light-emitting device of one embodimentof the present invention may be used for the display portion 3504.

FIG. 35C is a perspective view of a security light 3600. The securitylight 3600 includes lighting 3608 on the outside of the housing 3602,and a speaker 3610 and the like are incorporated in the housing 3602.The light-emitting device of one embodiment of the present invention canbe used for the lighting 3608.

The security light 3600 emits light when the lighting 3608 is gripped orheld, for example. An electronic circuit that can control the manner oflight emission from the security light 3600 may be provided in thehousing 3602. The electronic circuit may be a circuit that enables lightemission once or intermittently a plurality of times or may be a circuitthat can adjust the amount of emitted light by controlling the currentvalue for light emission. A circuit with which a loud audible alarm isoutput from the speaker 3610 at the same time as light emission from thelighting 3608 may be incorporated.

The security light 3600 can emit light in various directions; therefore,it is possible to intimidate a thug or the like with light, or light andsound. Moreover, the security light 3600 may include a camera such as adigital still camera to have a photography function.

FIG. 36 illustrates an example in which the light-emitting element isused for an indoor lighting device 8501. Since the light-emittingelement can have a larger area, a lighting device having a large areacan also be formed. In addition, a lighting device 8502 in which alight-emitting region has a curved surface can also be formed with theuse of a housing with a curved surface. A light-emitting elementdescribed in this embodiment is in the form of a thin film, which allowsthe housing to be designed more freely. Therefore, the lighting devicecan be elaborately designed in a variety of ways. Furthermore, a wall ofthe room may be provided with a large-sized lighting device 8503. Touchsensors may be provided in the lighting devices 8501, 8502, and 8503 tocontrol the power on/off of the lighting devices.

Moreover, when the light-emitting element is used on the surface side ofa table, a lighting device 8504 which has a function as a table can beobtained. When the light-emitting element is used as part of otherfurniture, a lighting device which has a function as the furniture canbe obtained.

As described above, lighting devices and electronic devices can beobtained by application of the light-emitting device of one embodimentof the present invention. Note that the light-emitting device can beused for electronic devices in a variety of fields without being limitedto the lighting devices and the electronic devices described in thisembodiment.

The structure described in this embodiment can be used in combinationwith any of the structures described in the other embodiments asappropriate.

Example 1

In this example, examples of fabricating light-emitting elements ofembodiments of the present invention are described. FIG. 37 is aschematic cross-sectional view of each of the light-emitting elementsfabricated in this example, and Table 1 shows details of the elementstructures. In addition, structures and abbreviations of compounds usedhere are given below.

TABLE 1 Thickness Weight Layer Symbol (nm) Material ratio Light-emittingElectrode 102 200 Al — element 1 Electron-injection layer 119 1 LiF —Electron-transport layer   118(2) 10 BPhen —   118(1) 20 4,6mCzP2Pm —Light-emitting layer 160 40 PCCzPTzn:Ir(tBuppm)₂(acac)  1:0.06Hole-transport layer 112 20 BPAFLP — Hole-injection layer 111 60DBT3P-II:MoO₃ 1:0.5 Electrode 101 70 ITSO — Light-emitting Electrode 102200 Al — element 2 Electron-injection layer 119 1 LiF —Electron-transport layer   118(2) 10 BPhen —   118(1) 20 4,6mCzP2Pm —Light-emitting layer 160 40 PCCzPTzn — Hole-transport layer 112 20BPAFLP — Hole-injection layer 111 60 DBT3P-II:MoO₃ 1:0.5 Electrode 10170 ITSO —

<Fabrication of Light-Emitting Elements> <<Fabrication of Light-EmittingElement 1>>

As the electrode 101, an ITSO film was formed to a thickness of 70 nmover the substrate 200. The electrode area of the electrode 101 was setto 4 mm² (2 mm×2 mm).

As the hole-injection layer 111,4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II) and molybdenum oxide (MoO₃) were deposited over the electrode101 by co-evaporation such that the deposited layer had a weight ratioof DBT3P-II: MoO₃=1:0.5 and a thickness of 60 nm.

As the hole-transport layer 112,4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP)was deposited over the hole-injection layer 111 by evaporation to athickness of 20 nm.

As a light-emitting layer 160,2-{4-[3-(N-phenyl-9H-carbazol-3-yl)-9H-carbazol-9-yl]phenyl}-4,6-diphenyl-1,3,5-triazine(abbreviation: PCCzPTzn) and(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)(abbreviation: Ir(tBuppm)₂(acac)) were deposited over the hole-transportlayer 112 by co-evaporation such that the deposited layer had a weightratio of PCCzPTzn: Ir(tBuppm)₂(acac)=1:0.06 and a thickness of 40 nm.Note that in the light-emitting layer 160, Ir(tBuppm)₂(acac) correspondsto a guest material and PCCzPTzn corresponds to a host material.

As the electron-transport layer 118,4,6-bis[3-(9H-carbazol-9-yl)phenyl]pyrimidine (abbreviation: 4,6mCzP2Pm)and bathophenanthroline (abbreviation: BPhen) were successivelydeposited by evaporation to thicknesses of 20 nm and 10 nm,respectively, over the light-emitting layer 160. As theelectron-injection layer 119, lithium fluoride (LiF) was deposited overthe electron-transport layer 118 by evaporation to a thickness of 1 nm.

As the electrode 102, aluminum (Al) was formed over theelectron-injection layer 119 to a thickness of 200 nm.

Next, in a glove box containing a nitrogen atmosphere, thelight-emitting element 1 was sealed by fixing the substrate 220 to thesubstrate 200 over which the organic material was deposited using asealant for an organic EL device. Specifically, after the sealant wasapplied to surround the organic material over the substrate 200 and thesubstrate 200 was bonded to the substrate 220, irradiation withultraviolet light having a wavelength of 365 nm at 6 J/cm² and heattreatment at 80° C. for one hour were performed. Through the abovesteps, the light-emitting element 1 was obtained.

<<Fabrication of Light-Emitting Element 2>>

For comparison, a light-emitting element 2 in which a guest material wasnot included and PCCzPTzn was included as a light-emitting material wasfabricated. The light-emitting element 2 was fabricated through the samesteps as those for the light-emitting element 1 except for the step offorming the light-emitting layer 160.

As the light-emitting layer 160 of the light-emitting element 2,PCCzPTzn was deposited by evaporation to a thickness of 40 nm.

<Characteristics of Light-Emitting Elements>

Then, the characteristics of the fabricated light-emitting elements 1and 2 were measured. Luminances and CIE chromaticities were measuredwith a luminance colorimeter (BM-5A manufactured by TOPCON TECHNOHOUSECORPORATION), and electroluminescence spectra were measured with amulti-channel spectrometer (PMA-11 manufactured by Hamamatsu PhotonicsK.K.).

FIG. 38 shows current efficiency vs. luminance characteristics of thelight-emitting elements 1 and 2; FIG. 39 shows luminance vs. voltagecharacteristics thereof, FIG. 40 shows external quantum efficiency vs.luminance characteristics thereof, and FIG. 41 shows power efficiencyvs. luminance characteristics thereof. The measurement for thelight-emitting elements was performed at room temperature (in anatmosphere kept at 23° C.).

Table 2 shows element characteristics of the light-emitting elements 1and 2 at around 1000 cd/m².

TABLE 2 External Current CIE Current Power quantum Voltage densityChromaticity Luminance efficiency efficiency efficiency (V) (mA/cm²) (x,y) (cd/m²) (cd/A) (lm/W) (%) Light-emitting 2.70 1.56 (0.423, 0.569)1190 76.4 88.9 21.1 element 1 Light-emitting 3.00 5.23 (0.265, 0.458) 972 18.6 19.5 7.08 element 2

FIG. 42 shows emission spectra when a current at a current density of2.5 mA/cm² was supplied to the light-emitting elements 1 and 2.

As shown in FIG. 38 to FIG. 41 and Table 2, the light-emitting element 1has high current efficiency and high external quantum efficiency, andthe external quantum efficiency of the light-emitting element 1 ishigher than 21%, which is an excellent value.

As shown in FIG. 42, the light-emitting element 1 emits green light. Theelectroluminescence spectrum of the light-emitting element 1 has a peakat a wavelength of 547 nm and a full width at half maximum of 77 nm.Note that the emission spectrum of the light-emitting element 2 has afull width at half maximum of 111 nm, which is wide. Thus, thelight-emitting element 1 including a guest material exhibits highercolor purity and better chromaticity than the light-emitting element 2.

The light-emitting element 1 was driven at an extremely low voltage of2.7 V at around cd/m² and thus exhibited high power efficiency.Furthermore, the light emission start voltage (voltages at the time whenthe luminance exceeds 1 cd/m²) of the light-emitting element 1 was 2.4V. The voltage is lower than a voltage corresponding to the energydifference between the LUMO level and the HOMO level of the guestmaterial Ir(tBuppm)₂(acac), which is described later. The resultssuggest that emission in the light-emitting element 1 is obtained not bydirect recombination of carriers in the guest material but byrecombination of carriers in the host material having a smaller energygap.

<Emission Spectra of Host Material>

In the fabricated light-emitting element 1, PCCzPTzn was used as thehost material. FIG. 43 shows measurement results of emission spectra ofa thin film of PCCzPTzn.

For the emission spectra measurement, a thin film sample was formed overa quartz substrate by a vacuum evaporation method. The emission spectrameasurement was performed with a PL microscope, LabRAM HR-PL, producedby HORIBA, Ltd., a He—Cd laser (wavelength: 325 nm) as excitation light,and a CCD detector, at a measurement temperature of 10 K. The S1 leveland the T1 level were calculated from peaks (including shoulders) on theshortest wavelength sides and the rising portions on the shorterwavelength sides of the emission spectra obtained by the measurement.The sample used for the measurement was fabricated as follows: a 50-nmthin film was formed over a quartz substrate, and, to the quartzsubstrate, another quartz substrate was attached from the film formationsurface side in a nitrogen atmosphere.

Note that in the measurement of the emission spectra, in addition to themeasurement of a normal emission spectrum, the measurement of atime-resolved emission spectrum in which light emission with a longlifetime is focused on was also performed. Since in this measurementmethod of emission spectra, the measurement temperature was set at a lowtemperature (10K), in the measurement of the normal emission spectrum,in addition to fluorescence, which is the main emission component,phosphorescence was observed. Furthermore, in the measurement of thetime-resolved emission spectrum in which light emission with a longlifetime is focused on, phosphorescence was mainly observed. That is, inthe measurement of the normal emission spectrum, fluorescent componentsof light were mainly observed, and, in the measurement of thetime-resolved emission spectrum, phosphorescent components of light weremainly observed.

As shown in FIG. 43, the wavelengths of peaks (including shoulders) onthe shortest wavelength sides of the emission spectra of PCCzPTzn thatindicate fluorescent components and phosphorescent components are 472 nmand 491 nm, respectively. Thus, the S1 level and the T1 level calculatedfrom the wavelengths of the peaks (including shoulders) are 2.63 eV and2.53 eV, respectively. That is, the energy difference between the S1level and the T1 level of PCCzPTzn calculated from the wavelengths ofthe peaks (including shoulders) was 0.1 eV, which is extremely small.

Furthermore, as shown in FIG. 43, the wavelengths of the rising portionson the shorter wavelength sides of the emission spectra of PCCzPTzn thatindicate fluorescent components and phosphorescent components are 450 nmand 477 nm, respectively. Thus, the S1 level and the T1 level calculatedfrom the wavelengths of the rising portions are 2.76 eV and 2.60 eV,respectively. That is, the energy difference between the S1 level andthe T1 level calculated from the wavelengths of the rising portions ofthe emission spectra of PCCzPTzn is 0.16 eV, which is also extremelysmall. Note that the wavelength of the rising portion on the shorterwavelength side of the emission spectrum is a wavelength at theintersection of the horizontal axis and a tangent to the spectrum at apoint where the slope of the tangent has a maximum value.

As described above, the energy difference between the S1 level and theT1 level of PCCzPTzn which is calculated from the wavelengths of thepeaks (including shoulders) on the shortest wavelength sides of theemission spectra and the energy difference between the S1 level and theT1 level of PCCzPTzn which is calculated from the wavelengths of therising portions on the shorter wavelength sides thereof are each greaterthan 0 eV and less than or equal to 0.2 eV, which is extremely small.Therefore, PCCzPTzn can have a function of converting triplet excitationenergy into singlet excitation energy by reverse intersystem crossing.

The peak wavelength on the shortest wavelength side of the emissionspectrum of light emission of PCCzPTzn that indicates phosphorescentcomponents is shorter than that of the electroluminescence spectrum ofthe guest material (Ir(tBuppm)₂(acac)) of the light-emitting element 1.Since Ir(tBuppm)₂(acac) serving as a guest material is a phosphorescentmaterial, light is emitted from the triplet excited state. That is, theT1 level of PCCzPTzn is higher than the T1 level of the guest material.

In addition, as described later, an absorption band on the lowest energyside (the longest wavelength side) of an absorption spectrum ofIr(tBuppm)₂(acac) is at around 500 nm and has a region overlapping withthe emission spectrum of PCCzPTzn. Therefore, in the light-emittingelement 1 using PCCzPTzn as a host material, excitation energy can beeffectively transferred from the host material to the guest material.

<Transient Fluorescent Characteristics of Host Material>

Next, transient fluorescent characteristics of PCCzPTzn were measuredusing time-resolved emission measurement.

The time-resolved emission measurement was performed on a thin-filmsample in which PCCzPTzn was deposited over a quartz substrate to athickness of 50 nm.

A picosecond fluorescence lifetime measurement system (manufactured byHamamatsu Photonics K.K.) was used for the measurement. In thismeasurement, the thin film was irradiated with pulsed laser, andemission of the thin film which was attenuated from the laserirradiation underwent time-resolved measurement using a streak camera tomeasure the lifetime of fluorescent emission of the thin film. Anitrogen gas laser with a wavelength of 337 nm was used as the pulsedlaser. The thin film was irradiated with pulsed laser with a pulse widthof 500 ps at a repetition rate of 10 Hz. By integrating data obtained bythe repeated measurement, data with a high S/N ratio was obtained. Themeasurement was performed at room temperature (in an atmosphere kept at23° C.).

FIG. 44 shows transient fluorescent characteristics of PCCzPTzn obtainedby the measurement.

The attenuation curve shown in FIG. 44 was fitted with Formula 4.

$\begin{matrix}{L = {\sum\limits_{n = 1}{A_{n}{\exp \left( {- \frac{t}{a_{n}}} \right)}}}} & \left\lbrack {{Formula}\mspace{14mu} 4} \right\rbrack\end{matrix}$

In Formula 4, L and t represent normalized emission intensity andelapsed time, respectively. This fitting results show that the emissioncomponent of the PCCzPTzn thin-film sample contains at least afluorescent component having an emission lifetime of 0.015 μs and adelayed fluorescence component having an emission lifetime of 1.5 μs. Inother words, it is found that PCCzPTzn is a thermally activated delayedfluorescent material exhibiting delayed fluorescent at room temperature.

As shown in FIG. 38 to FIG. 41 and Table 2, it is found that the maximumexternal quantum efficiency of the light-emitting element 2 is 8.6%,which is a high value, though the light-emitting element 2 does notinclude a phosphorescent material as a guest material. Since the maximumprobability of formation of singlet excitons by recombination ofcarriers (holes and electrons) injected from a pair of electrodes is25%, the maximum external quantum efficiency in the case where the lightextraction efficiency to the outside is 25% is 6.25%. The reason why theexternal quantum efficiency of the light-emitting element 2 is higherthan 6.25% is that, as described above, PCCzPTzn is a material having asmall energy difference between the S1 level and the T1 level andexhibiting thermally activated delayed fluorescence, and therefore has afunction of emitting light originating from singlet excitons generatedby reverse intersystem crossing from triplet excitons as well as lightoriginating from singlet excitons generated by recombination of carriers(holes and electrons) injected from the pair of electrodes.

Meanwhile, as shown in FIG. 42, the wavelength of a peak of theelectroluminescence spectrum of the light-emitting element 2 is 507 nm,which is shorter than the wavelength of the peak of theelectroluminescence spectrum of the light-emitting element 1. Theelectroluminescence spectrum of the light-emitting element 1 indicateslight originating from phosphorescence of the guest material(Ir(tBuppm)₂(acac)). The electroluminescence spectrum of thelight-emitting element 2 indicates light originating from fluorescenceand thermally activated delayed fluorescence of PCCzPTzn. Note that asdescribed above, the energy difference between the S1 level and the T1level of PCCzPTzn is as small as 0.1 eV. Therefore, the above-describedmeasurement results of the electroluminescence spectra of thelight-emitting elements 1 and 2 also show that the T1 level of PCCzPTznis higher than the T1 level of the guest material (Ir(tBuppm)₂(acac))and PCCzPTzn can be suitably used as the host material of thelight-emitting element 1.

<Results of CV Measurement>

The electrochemical characteristics (oxidation reaction characteristicsand reduction reaction characteristics) of the compounds used as theguest material and the host material of the light-emitting element 1were examined by cyclic voltammetry (CV). Note that for the measurement,an electrochemical analyzer (ALS model 600A or 600C, produced by BASInc.) was used, and measurement was performed on a solution obtained bydissolving each compound in N,N-dimethylformamide (abbreviation: DMF).In the measurement, the potential of a working electrode with respect tothe reference electrode was changed within an appropriate range, so thatthe oxidation peak potential and the reduction peak potential wereobtained. In addition, the HOMO and LUMO levels of each compound werecalculated from the estimated redox potential of the reference electrodeof −4.94 eV and the obtained peak potentials.

Table 3 shows oxidation potentials and reduction potentials obtained byCV measurement and HOMO levels and LUMO levels of the compoundscalculated from the CV measurement results.

TABLE 3 HOMO level LUMO level calculated from calculated from OxidationReduction oxidation reduction Abbreviation potential (V) potential (V)potential (eV) potential (eV) Ir(tBuppm)2(acac) 0.62 −2.21 −5.56 −2.73PCCzPTzn 0.70 −1.97 −5.64 −2.97

As shown in Table 3, in the light-emitting element 1, the reductionpotential of the guest material (Ir(tBuppm)₂(acac)) is lower than thereduction potential of the host material (PCCzPTzn), and the oxidationpotential of the guest material (Ir(tBuppm)₂(acac)) is lower than theoxidation potential of the host material (PCCzPTzn). Therefore, the LUMOlevel of the guest material (Ir(tBuppm)₂(acac)) is higher than the LUMOlevel of the host material (PCCzPTzn), and the HOMO level of the guestmaterial (Ir(tBuppm)₂(acac)) is higher than the HOMO level of the hostmaterial (PCCzPTzn). The energy difference between the LUMO level andthe HOMO level of the guest material (Ir(tBuppm)₂(acac)) is larger thanthe energy difference between the LUMO level and the HOMO level of thehost material (PCCzPTzn).

<Absorption Spectrum and Emission Spectrum of Guest Material>

FIG. 45 shows the measurement results of the absorption spectrum andemission spectrum of Ir(tBuppm)₂(acac) that is the guest material in thelight-emitting element 1.

For the measurement of the absorption spectrum and emission spectrum, adichloromethane solution in which Ir(tBuppm)₂(acac) was dissolved wasprepared, and a quartz cell was used. The absorption spectrum wasmeasured using an ultraviolet-visible spectrophotometer (V-550, producedby JASCO Corporation). Then, the absorption spectrum of a quartz cellwas subtracted from the measured spectrum of the sample. Note that theemission spectrum of the solution was measured with a PL-EL measurementapparatus (manufactured by Hamamatsu Photonics K.K.). The measurementwas performed at room temperature (in an atmosphere kept at 23° C.).

As shown in FIG. 45, the absorption band on the lowest energy side (thelongest wavelength side) of the absorption spectrum of Ir(tBuppm)₂(acac)is at around 500 nm. The absorption edge was obtained from data of theabsorption spectrum, and the transition energy was estimated on theassumption of direct transition. As a result, the absorption edge ofIr(tBuppm)₂(acac) was 526 nm and the transition energy was calculated tobe 2.36 eV.

The energy difference between the LUMO level and the HOMO level ofIr(tBuppm)₂(acac) was 2.83 eV. This value was calculated from the CVmeasurement results shown in Table 3.

That is, the energy difference between the LUMO level and the HOMO levelof Ir(tBuppm)₂(acac) is larger than the transition energy thereofcalculated from the absorption edge of the absorption spectrum by 0.47eV.

As shown in FIG. 42, the wavelength of the peak on the shortestwavelength side of the electroluminescence spectrum of thelight-emitting element 1 is 547 nm. According to that, the lightemission energy of Ir(tBuppm)₂(acac) was calculated to be 2.27 eV.

That is, the energy difference between the LUMO level and the HOMO levelof Ir(tBuppm)₂(acac) was larger than the light emission energy by 0.56eV.

Consequently, in the guest material of the light-emitting element 1, theenergy difference between the LUMO level and the HOMO level is greaterthan the transition energy calculated from the absorption edge by 0.4 eVor more. In addition, the energy difference between the LUMO level andthe HOMO level is greater than the light emission energy by 0.4 eV ormore. Therefore, high energy corresponding to the energy differencebetween the LUMO level and the HOMO level is needed, that is, highvoltage is needed when carriers injected from a pair of electrodes aredirectly recombined in the guest material.

Meanwhile, the energy difference between the LUMO level and the HOMOlevel of the host material (PCCzPTzn) in the light-emitting element 1was calculated to be 2.67 eV from Table 3. That is, the energydifference between the LUMO level and the HOMO level of the hostmaterial (PCCzPTzn) of the light-emitting element 1 is smaller than theenergy difference (2.83 eV) between the LUMO level and the HOMO level ofthe guest material (Ir(tBuppm)₂(acac)), greater than the transitionenergy (2.36 eV) calculated from the absorption edge, and greater thanthe light emission energy (2.27 eV). Therefore, in the light-emittingelement 1, the guest material can be excited by energy transfer throughan excited state of the host material without the direct carrierrecombination in the guest material, whereby the driving voltage can belowered. Thus, the power consumption of the light-emitting element ofone embodiment of the present invention can be reduced.

According to the CV measurement results in Table 3, among carriers(electrons and holes) injected from the pair of electrodes of thelight-emitting element 1, electrons tend to be injected into the hostmaterial (PCCzPTzn) with a low LUMO level, whereas holes tend to beinjected into the guest material (Ir(tBuppm)₂(acac)) with a high HOMOlevel. That is, there is a possibility that an exciplex is formed by thehost material and the guest material.

The energy difference between the LUMO level of the host material(PCCzPTzn) and the HOMO level of the guest material (Ir(tBuppm)₂(acac))was calculated from the CV measurement results shown in Table 3 andfound to be 2.59 eV.

From these results, in the light-emitting element 1, the energydifference (2.59 eV) between the LUMO level of the host material(PCCzPTzn) and the HOMO level of the guest material (Ir(tBuppm)₂(acac))is greater than or equal to the transition energy (2.36 eV) calculatedfrom the absorption edge of the absorption spectrum of the guestmaterial. Furthermore, the energy difference (2.59 eV) between the LUMOlevel of the host material and the HOMO level of the guest material isgreater than or equal to the energy (2.27 eV) of light emitted by theguest material. Accordingly, rather than formation of an exciplex by thehost material and the guest material, transfer of excitation energy tothe guest material is more facilitated eventually, whereby efficientlight emission from the guest material is achieved. This relationship isa feature of one embodiment of the present invention for efficient lightemission.

In the case where the HOMO level of a guest material is higher than theHOMO level of a host material and the energy difference between the LUMOlevel and the HOMO level of the guest material is larger than the energydifference between the LUMO level and the HOMO level of the hostmaterial as in the above-described light-emitting element 1, alight-emitting element with high emission efficiency and low drivingvoltage can be obtained when the energy difference between the LUMOlevel of the host material and the HOMO level of the guest material isgreater than or equal to the transition energy calculated from theabsorption edge of the absorption spectrum of the guest material orgreater than or equal to the light emission energy of the guestmaterial. Furthermore, in the case where the energy difference betweenthe LUMO level and the HOMO level of a guest material is greater thanthe transition energy calculated from the absorption edge of theabsorption spectrum of the guest material or the light emission energyof the guest material by 0.4 eV or more, a light-emitting element withhigh emission efficiency and low driving voltage can be obtained.

As described above, by employing the structure of one embodiment of thepresent invention, a light-emitting element having high emissionefficiency can be fabricated. Furthermore, a light-emitting element withreduced power consumption can be fabricated.

The structures described in this example can be used in an appropriatecombination with any of the other embodiments and examples.

Example 2

In this example, examples of fabricating light-emitting elements ofembodiments of the present invention (a light-emitting element 3 and alight-emitting element 4) and a comparative light-emitting element (acomparative light-emitting element 1) are described. Schematiccross-sectional views of the light-emitting elements fabricated in thisexample are similar to those shown in FIG. 37. Table 4 and Table 5 showdetails of the element structures. In addition, structures andabbreviations of compounds used here are given below. Note that theabove example can be referred to for other compounds.

TABLE 4 Thickness Weight Layer Symbol (nm) Material ratio Light-Electrode 102 200 Al — emitting Electron-injection layer 119 1 LiF —element 3 Electron-transport layer   118(2) 15 BPhen —   118(1) 10PCCzPTzn — Light-emitting layer 160 40 PCCzPTzn:Ir(mpptz-  1:0.06diBuCNp)₃ Hole-transport layer 112 20 PCCP — Hole-injection layer 111 20DBT3P-II:MoO₃ 1:0.5 Electrode 101 70 ITSO — Light- Electrode 102 200 Al— emitting Electron-injection layer 119 1 LiF — element 4Electron-transport layer   118(2) 15 BPhen —   118(1) 10 PCCzPTzn —Light-emitting layer   160(2) 20 PCCzPTzn:PCCP:Ir(mpptz- 0.85:0.15:0.06diBuCNp)₃   160(1) 20 PCCzPTzn:PCCP:Ir(mpptz- 0.75:0.25:0.06 diBuCNp)₃Hole-transport layer 112 20 PCCP — Hole-injection layer 111 20DBT3P-II:MoO₃ 1:0.5 Electrode 101 70 ITSO —

TABLE 5 Thickness Weight Layer Symbol (nm) Material ratio ComparativeElectrode 102 200 Al — light-emitting Electron-injection layer 119 1 LiF— element 1 Electron-transport layer 118 30 BPhen — Light-emitting layer160 30 Cz2DBT:PCCzPTzn 0.9:0.1 Hole-transport layer 112 20 Cz2DBT —Hole-injection layer 111 60 DBT3P-II:MoO₃  1:0.5 Electrode 101 110 ITSO—

<Fabrication of Light-Emitting Elements> <<Fabrication of Light-EmittingElement 3>>

As the electrode 101, an ITSO film was formed to a thickness of 70 nmover the substrate 200. The electrode area of the electrode 101 was setto 4 mm² (2 mm×2 mm).

As the hole-injection layer 111, DBT3P-II and MoO₃ were deposited overthe electrode by co-evaporation such that the deposited layer had aweight ratio of DBT3P-II: MoO₃=1:0.5 and a thickness of 20 nm.

As the hole-transport layer 112, 3,3′-bis(9-phenyl-9H-carbazole)(abbreviation: PCCP) was deposited over the hole-injection layer 111 byevaporation to a thickness of 20 nm.

As the light-emitting layer 160, PCCzPTzn andtris{2-[4-(4-cyano-2,6-diisobutylphenyl)-5-(2-methylphenyl)-4H-1,2,4-triazol-3-yl-κN²]phenyl-κC}iridium(III)(abbreviation: Ir(mpptz-diBuCNp)₃) were deposited over thehole-transport layer by co-evaporation such that the deposited layer hada weight ratio of PCCzPTzn: Ir(mpptz-diBuCNp)₃=1:0.06 and a thickness of40 nm. Note that in the light-emitting layer 160, Ir(mpptz-diBuCNp)₃corresponds to a guest material and PCCzPTzn corresponds to a hostmaterial.

As the electron-transport layer 118, PCCzPTzn and BPhen weresuccessively deposited by evaporation to thicknesses of 10 nm and 15 nm,respectively, over the light-emitting layer 160. As theelectron-injection layer 119, lithium fluoride (LiF) was deposited overthe electron-transport layer 118 by evaporation to a thickness of 1 nm.

As the electrode 102, aluminum (Al) was formed over theelectron-injection layer 119 to a thickness of 200 nm.

Next, in a glove box containing a nitrogen atmosphere, thelight-emitting element 3 was sealed by fixing the substrate 220 to thesubstrate 200 over which the organic material was deposited using asealant for an organic EL device. For the detailed method, descriptionof the light-emitting element 1 can be referred to.

<<Fabrication of Light-Emitting Element 4>>

The light-emitting element 4 was fabricated through the same steps asthose for the light-emitting element 3 except for the step of formingthe light-emitting layer 160.

As the light-emitting layer 160 of the light-emitting element 4,PCCzPTzn, PCCP, and Ir(mpptz-diBuCNp)₃ were deposited by co-evaporationsuch that the deposited layer had a weight ratio of PCCzPTzn: PCCP:Ir(mpptz-diBuCNp)₃=0.75:0.25:0.06 and a thickness of 20 nm, and then,PCCzPTzn, PCCP, and Ir(mpptz-diBuCNp)₃ were deposited by co-evaporationsuch that the deposited layer had a weight ratio of PCCzPTzn: PCCP:Ir(mpptz-diBuCNp)₃=0.85:0.15:0.06 and a thickness of 20 nm. Note that inthe light-emitting layer 160, Ir(mpptz-diBuCNp)₃ corresponds to a guestmaterial, PCCzPTzn corresponds to a host material, and PCCP correspondsto a material for adjusting carrier balance.

<<Fabrication of Comparative Light-Emitting Element 1>>

As the electrode 101, an ITSO film was formed to a thickness of 110 nmover the substrate 200. The electrode area of the electrode 101 was setto 4 mm² (2 mm×2 mm).

As the hole-injection layer 111, DBT3P-II and MoO₃ were deposited overthe electrode by co-evaporation such that the deposited layer had aweight ratio of DBT3P-II: MoO₃=1:0.5 and a thickness of 60 nm. As thehole-transport layer 112, 2,8-di(9H-carbazol-9-yl)-dibenzothiophene(abbreviation: Cz2DBT) was deposited over the hole-injection layer 111by evaporation to a thickness of 20 nm.

As the light-emitting layer 160, Cz2DBT and PCCzPTzn were deposited overthe hole-transport layer 112 by co-evaporation such that the depositedlayer had a weight ratio of Cz2DBT: PCCzPTzn=0.9:0.1 and a thickness of30 nm.

As the electron-transport layer 118, BPhen was deposited by evaporationto a thickness of 30 nm over the light-emitting layer 160. As theelectron-injection layer 119, LiF was deposited over theelectron-transport layer 118 by evaporation to a thickness of 1 nm.

As the electrode 102, aluminum (Al) was deposited over theelectron-injection layer 119 to a thickness of 200 nm.

Next, in a glove box containing a nitrogen atmosphere, the comparativelight-emitting element 1 was sealed by fixing the substrate 220 to thesubstrate 200 over which the organic material was deposited using asealant for an organic EL device. For the detailed method, descriptionof the light-emitting element 1 can be referred to. Through the abovesteps, the comparative light-emitting element 1 was obtained.

<Characteristics of Light-Emitting Elements>

FIG. 46 shows current efficiency vs. luminance characteristics of thelight-emitting elements 3 and 4; FIG. 47 shows luminance vs. voltagecharacteristics thereof, FIG. 48 shows external quantum efficiency vs.luminance characteristics thereof, and FIG. 49 shows power efficiencyvs. luminance characteristics thereof. Note that the measurement for thelight-emitting elements was performed at room temperature (in anatmosphere kept at 23° C.) by a measurement method similar to that usedin Example 1.

Table 6 shows element characteristics of the light-emitting elements 3and 4 at around cd/m².

TABLE 6 External Current CIE Current Power quantum Voltage densityChromaticity Luminance efficiency efficiency efficiency (V) (mA/cm²) (x,y) (cd/m²) (cd/A) (lm/W) (%) Light-emitting 2.70 1.52 (0.206, 0.517) 828 54.3 63.2 20.7 element 3 Light-emitting 2.80 1.76 (0.202, 0.513)1110 63.1 70.8 24.2 element 4

FIG. 50 shows emission spectra of the light-emitting elements 3 and 4when a current at a current density of 2.5 mA/cm² was supplied to thelight-emitting elements 3 and 4.

As shown in FIG. 46 to FIG. 49 and Table 6, the light-emitting elements3 and 4 have high current efficiency and high external quantumefficiency. In addition, the maximum external quantum efficiency of thelight-emitting element 4 is 24.8%, which is an excellent value. Thereason why the light-emitting element 4 has higher efficiency than thelight-emitting element 3 is that the carrier balance is improved by PCCPincluded in the light-emitting layer of the light-emitting element 4.

Moreover, as shown in FIG. 50, the electroluminescence spectra of thelight-emitting elements 3 and 4 largely overlap with each other and arealmost the same. The light-emitting element 3 emits blue light. Theelectroluminescence spectrum of the light-emitting element 3 has a peakat a wavelength of 499 nm and a full width at half maximum of 71 nm.

The light-emitting elements 3 and 4 were driven at an extremely lowvoltage of 3 V or less at around 1000 cd/m² and thus exhibited highpower efficiency. Furthermore, the light emission start voltage(voltages at the time when the luminance exceeds 1 cd/m²) of thelight-emitting elements 3 and 4 was 2.3 V. The voltage is lower than avoltage corresponding to the energy difference between the LUMO leveland the HOMO level of the guest material Ir(mpptz-diBuCNp)₃, which isdescribed later. The results suggest that emission of the light-emittingelements 3 and 4 is obtained not by direct recombination of carriers inthe guest material but by recombination of carriers in the materialhaving a smaller energy gap.

As shown in FIG. 43 in Example 1, the peak wavelength (491 nm) on theshortest wavelength side of the emission spectrum of light emission ofthe thin film of PCCzPTzn (i.e., the host material in the fabricatedlight-emitting elements 3 and 4) that indicates phosphorescentcomponents is shorter than that of the electroluminescence spectrum ofthe guest material (Ir(mpptz-diBuCNp)₃) of the light-emitting elements 3and 4. Since Ir(mpptz-diBuCNp)₃ serving as a guest material is aphosphorescent material, light is emitted from the triplet excitedstate. That is, the triplet excitation energy of PCCzPTzn is higher thanthe triplet excitation energy of the guest material.

In addition, as described later, an absorption band on the lowest energyside (the longest wavelength side) of an absorption spectrum ofIr(mpptz-diBuCNp)₃ is at around 450 nm and has a region overlapping withthe emission spectrum of PCCzPTzn. Therefore, in the light-emittingelement using PCCzPTzn as a host material, excitation energy can beeffectively transferred to the guest material.

As shown in FIG. 43, PCCzPTzn is a thermally activated delayedfluorescence substance exhibiting delayed fluorescent at roomtemperature.

<Characteristics of Comparative Light-Emitting Element>

FIG. 51 shows current efficiency vs. luminance characteristics of thecomparative light-emitting element 1 in which PCCzPTzn is used as alight-emitting material; FIG. 52 shows luminance vs. voltagecharacteristics thereof, FIG. 53 shows external quantum efficiency vs.luminance characteristics thereof, and FIG. 54 shows power efficiencyvs. luminance characteristics thereof. Note that the measurement wasperformed at room temperature (in an atmosphere kept at 23° C.).

Table 7 shows the element characteristics of the comparativelight-emitting element 1 at around 1000 cd/m².

TABLE 7 External Current CIE Current Power quantum Voltage densityChromaticity Luminance efficiency efficiency efficiency (V) (mA/cm²) (x,y) (cd/m²) (cd/A) (lm/W) (%) Comparative light- 4.00 4.73 (0.186, 0.284)1010 21.4 16.8 11.9 emitting element 1

FIG. 55 shows an emission spectrum of the comparative light-emittingelement 1 when a current with a current density of 2.5 mA/cm² wassupplied to the comparative light-emitting element 1.

As shown in FIG. 51 to FIG. 54 and Table 7, the comparativelight-emitting element 1 has high current efficiency and high externalquantum efficiency. The maximum external quantum efficiency of thecomparative light-emitting element 1 is 23.4%, which is an excellentvalue. Since the maximum probability of formation of singlet excitons byrecombination of carriers (holes and electrons) injected from a pair ofelectrodes is 25%, the maximum external quantum efficiency in the casewhere the light extraction efficiency to the outside is 25% is 6.25%.The reason why the external quantum efficiency of the comparativelight-emitting element 1 is higher than 6.25% is that, as describedabove, PCCzPTzn is a material having a small difference between thesinglet excitation energy level and the triplet excitation energy leveland exhibiting thermally activated delayed fluorescence, and has afunction of emitting light originating from singlet excitons generatedby reverse intersystem crossing from triplet excitons as well as lightoriginating from singlet excitons generated by recombination of carriers(holes and electrons) injected from the pair of electrodes.

Meanwhile, as shown in FIG. 55, the peak wavelength of theelectroluminescence spectrum of the comparative light-emitting element 1is 472 nm, which is shorter than the peak wavelengths of theelectroluminescence spectra of the light-emitting elements 3 and 4. Theelectroluminescence spectra of the light-emitting elements 3 and 4indicate light originating from phosphorescence of the guest material(Ir(mpptz-diBuCNp)₃). The electroluminescence spectrum of thecomparative light-emitting element 1 indicates light originating fromfluorescence and thermally activated delayed fluorescence of PCCzPTzn.Note that as described in the above example, the energy differencebetween the S1 level and the T1 level of PCCzPTzn is as small as 0.1 eV.Therefore, the above-described measurement results of theelectroluminescence spectra of the light-emitting elements 3 and 4 andthe comparative light-emitting element 1 also show that the T1 level ofPCCzPTzn is higher than the T1 level of the guest material(Ir(mpptz-diBuCNp)₃) and PCCzPTzn can be suitably used as the hostmaterials of the light-emitting elements 3 and 4.

<Results of CV Measurement>

The electrochemical characteristics (oxidation reaction characteristicsand reduction reaction characteristics) of the compounds used as theguest material and the host material of the light-emitting elements wereexamined by cyclic voltammetry (CV). The measurement method was similarto that used in Example 1.

For the measurement of oxidation reaction characteristics and reductionreaction characteristics of PCCzPTzn and PCCP, a solution obtained bydissolving the material in N,N-dimethylformamide (abbreviation: DMF) wasused. In general, an organic compound used in an organic EL element hasa refractive index of approximately 1.7 to 1.8 and its relativedielectric constant is approximately 3. When DMF, which is a highpolarity solvent (relative dielectric constant: 38), is used formeasurement of oxidation reaction characteristics of a compoundincluding a substituent with a high polarity (in particular, with a highelectron-withdrawing property) such as a cyano group, the accuracy mightbe decreased. For this reason, in this example, a solution obtained bydissolving the guest material (Ir(mpptz-diBuCNp)₃) in chloroform with alow polarity (relative dielectric constant: 4.8) was used for themeasurement of oxidation reaction characteristics. For the measurementof reduction reaction characteristics of the guest material, a solutionobtained by dissolving the guest material in DMF was used.

Table 8 shows oxidation potentials and reduction potentials obtained byCV measurement and HOMO levels and LUMO levels of the compoundscalculated from the CV measurement results.

TABLE 8 HOMO level LUMO level calculated from calculated from OxidationReduction oxidation reduction Abbreviation potential (V) potential (V)potential (eV) potential (eV) Ir(mpptz-diBuCNp)₃ 0.46 −2.46 −5.40 −2.49PCCzPTzn 0.70 −1.97 −5.64 −2.97 PCCP 0.69 −2.98 −5.63 −1.96

As shown in Table 8, in the light-emitting elements 3 and 4, thereduction potential of the guest material (Ir(mpptz-diBuCNp)₃) is lowerthan the reduction potential of the host material (PCCzPTzn), and theoxidation potential of the guest material (Ir(mpptz-diBuCNp)₃) is lowerthan the oxidation potential of the host material (PCCzPTzn). Therefore,the LUMO level of the guest material (Ir(mpptz-diBuCNp)₃) is higher thanthe LUMO level of the host material (PCCzPTzn), and the HOMO level ofthe guest material (Ir(mpptz-diBuCNp)₃) is higher than the HOMO level ofthe host material (PCCzPTzn). The energy difference between the LUMOlevel and the HOMO level of the guest material (Ir(mpptz-diBuCNp)₃) islarger than the energy difference between the LUMO level and the HOMOlevel of the host material (PCCzPTzn).

Note that the reduction potential of PCCP is lower than that ofPCCzPTzn, and the oxidation potential of PCCP is equivalent to that ofPCCzPTzn. The LUMO level of PCCP is higher than that of PCCzPTzn, andthe HOMO level of PCCP is equivalent to that of PCCzPTzn. Therefore,PCCP has a function of transporting holes in the light-emitting layerincluding PCCzPTzn as a host material. Therefore, as compared to thelight-emitting element 3, the light-emitting element 4 has improvedcarrier balance and higher emission efficiency.

For the calculation of the triplet excitation energy level of PCCP, thephosphorescence spectrum was measured. The peak wavelength on theshortest wavelength side of the phosphorescence spectrum of PCCP was 467nm, and thus, the triplet excitation energy level was calculated to be2.66 eV. That is, PCCP was a material whose triplet excitation energylevel was higher than that of PCCzPTzn. Note that a measurement methodof the phosphorescence spectrum of PCCP was similar to theabove-described measurement method of the case of PCCzPTzn. The tripletexcitation energy level of PCCP was calculated from the peak wavelengthof the phosphorescence spectrum.

<Absorption Spectrum and Emission Spectrum of Guest Material>

FIG. 56 shows the measurement results of the absorption spectrum andemission spectrum of Ir(mpptz-diBuCNp)₃ that is the guest material inthe light-emitting element.

For the measurement of the absorption spectrum and emission spectrum, adichloromethane solution in which Ir(mpptz-diBuCNp)₃ was dissolved wasprepared, and a quartz cell was used. The absorption spectrum wasmeasured using an ultraviolet-visible spectrophotometer (V-550, producedby JASCO Corporation). Then, the absorption spectrum of a quartz cellwas subtracted from the measured spectrum of the sample. Note that theemission spectrum of the solution was measured with a PL-EL measurementapparatus (manufactured by Hamamatsu Photonics K.K.). The measurementwas performed at room temperature (in an atmosphere kept at 23° C.).

As shown in FIG. 56, the absorption band on the lowest energy side (thelongest wavelength side) of the absorption spectrum ofIr(mpptz-diBuCNp)₃ is at around 450 nm. The absorption edge was obtainedfrom data of the absorption spectrum, and the transition energy wasestimated on the assumption of direct transition. As a result, theabsorption edge of Ir(mpptz-diBuCNp)₃ was 478 nm and the transitionenergy was calculated to be 2.59 eV.

The energy difference between the LUMO level and the HOMO level ofIr(mpptz-diBuCNp)₃ was 2.92 eV. This value was calculated from the CVmeasurement results shown in Table 8.

That is, the energy difference between the LUMO level and the HOMO levelof Ir(mpptz-diBuCNp)₃ is greater than the transition energy thereofcalculated from the absorption edge by 0.33 eV.

As shown in FIG. 50, the wavelength of the peak on the shortestwavelength side of the electroluminescence spectrum of thelight-emitting element 3 is 499 nm. According to that, the lightemission energy of Ir(mpptz-diBuCNp)₃ was calculated to be 2.48 eV.

That is, the energy difference between the LUMO level and the HOMO levelof Ir(mpptz-diBuCNp)₃ was greater than the light emission energy by 0.44eV.

Consequently, in the guest material of the light-emitting element, theenergy difference between the LUMO level and the HOMO level is greaterthan the transition energy calculated from the absorption edge by 0.3 eVor more. In addition, the energy difference between the LUMO level andthe HOMO level is greater than the light emission energy by 0.4 eV ormore. Therefore, high energy corresponding to the energy differencebetween the LUMO level and the HOMO level is needed, that is, highvoltage is needed when carriers injected from a pair of electrodes aredirectly recombined in the guest material.

Meanwhile, the energy difference between the LUMO level and the HOMOlevel of the host material (PCCzPTzn) in the light-emitting elements 3and 4 was calculated to be 2.67 eV from Table 8. That is, the energydifference between the LUMO level and the HOMO level of the hostmaterial (PCCzPTzn) of the light-emitting elements 3 and 4 is smallerthan the energy difference (2.92 eV) between the LUMO level and the HOMOlevel of the guest material (Ir(mpptz-diBuCNp)₃), greater than thetransition energy (2.59 eV) calculated from the absorption edge, andgreater than the light emission energy (2.48 eV). Therefore, in thelight-emitting elements 3 and 4, the guest material can be excited byenergy transfer through an excited state of the host material withoutthe direct carrier recombination in the guest material, whereby thedriving voltage can be lowered. Thus, the power consumption of thelight-emitting element of one embodiment of the present invention can bereduced.

In the case where the HOMO level of a guest material is higher than theHOMO level of a host material and the energy difference between the LUMOlevel and the HOMO level of the guest material is larger than the energydifference between the LUMO level and the HOMO level of the hostmaterial as in the light-emitting elements 3 and 4, a light-emittingelement with high emission efficiency and low driving voltage can beobtained when the energy difference between the LUMO level of the hostmaterial and the HOMO level of the guest material is greater than orequal to the transition energy calculated from the absorption edge ofthe absorption spectrum of the guest material or the light emissionenergy of the guest material. Furthermore, in the case where the energydifference between the LUMO level and the HOMO level of a guest materialis greater than the transition energy calculated from the absorptionedge of the absorption spectrum of the guest material or greater than orequal to the light emission energy of the guest material by 0.3 eV ormore, a light-emitting element with high emission efficiency and lowdriving voltage can be obtained.

As described above, by employing the structure of one embodiment of thepresent invention, a light-emitting element having high emissionefficiency can be fabricated. Furthermore, a light-emitting element withreduced power consumption can be fabricated, and a light-emittingelement having high emission efficiency and emitting blue light can befabricated.

The structures described in this example can be used in an appropriatecombination with any of the other embodiments and examples.

Example 3

In this example, examples of fabricating a light-emitting element ofembodiments of the present invention (a light-emitting element 5) and acomparative light-emitting element (a comparative light-emitting element2) are described. Schematic cross-sectional views of the light-emittingelements fabricated in this example are similar to those shown in FIG.37. Table and Table 10 show details of the element structures. Inaddition, structures and abbreviations of compounds used here are givenbelow. Note that the above example can be referred to for othercompounds.

TABLE 9 Thickness Weight Layer Symbol (nm) Material ratio Light-Electrode 102 200 Al — emitting Electron-injection layer 119 1 LiF —element 5 Electron-transport layer   118(2) 15 BPhen —   118(1) 104,6mCzP2Pm — Light-emitting layer 160 40 4PCCzBfpm:Ir(mpptz- 1:0.06diBuCNp)₃ Hole-transport layer 112 20 PCCP — Hole-injection layer 111 15DBT3P-II:MoO₃ 1:0.5  Electrode 101 70 ITSO —

TABLE 10 Thickness Weight Layer Symbol (nm) Material ratio ComparativeElectrode 102 200 Al — light-emitting Electron-injection layer 119 1 LiF— element 2 Electron-transport layer   118(2) 40 TmPyPB —   118(1) 5DPEPO — Light-emitting layer 160 15 DPEPO:4PCCzBfpm 0.85:0.15Hole-transport layer 112 20 Cz2DBT — Hole-injection layer 111 20DBT3P-II:MoO₃  1:0.5 Electrode 101 70 ITSO —

<Fabrication of Light-Emitting Elements> <<Fabrication of Light-EmittingElement 5>>

As the electrode 101, an ITSO film was formed to a thickness of 70 nmover the substrate 200. The electrode area of the electrode 101 was setto 4 mm² (2 mm×2 mm).

As the hole-injection layer 111, DBT3P-II and MoO₃ were deposited overthe electrode by co-evaporation such that the deposited layer had aweight ratio of DBT3P-II: MoO₃=1:0.5 and a thickness of 15 nm.

As the hole-transport layer 112, PCCP was deposited over thehole-injection layer 111 by evaporation to a thickness of 20 nm.

As the light-emitting layer 160,4-(9′-phenyl-3,3′-bi-9H-carbazol-9-yl)benzofuro[3,2-d]pyrimidine(abbreviation: 4PCCzBfpm) and Ir(mpptz-diBuCNp)₃ were deposited over thehole-transport layer 112 by co-evaporation such that the deposited layerhad a weight ratio of 4PCCzBfpm: Ir(mpptz-diBuCNp)₃=1:0.06 and athickness of 40 nm. Note that in the light-emitting layer 160,Ir(mpptz-diBuCNp)₃ corresponds to a guest material and 4PCCzBfpmcorresponds to a host material.

As the electron-transport layer 118, 4,6mCzP2Pm and BPhen weresuccessively deposited by evaporation to thicknesses of 10 nm and 15 nm,respectively, over the light-emitting layer 160. As theelectron-injection layer 119, LiF was deposited over theelectron-transport layer 118 by evaporation to a thickness of 1 nm.

As the electrode 102, aluminum (Al) was formed over theelectron-injection layer 119 to a thickness of 200 nm.

Next, in a glove box containing a nitrogen atmosphere, thelight-emitting element 5 was sealed by fixing the substrate 220 to thesubstrate 200 over which the organic material was deposited using asealant for an organic EL device. For the detailed method, descriptionof the light-emitting element 1 can be referred to. Through the abovesteps, the light-emitting element 5 was obtained.

<<Fabrication of Comparative Light-Emitting Element 2>>

As the electrode 101, an ITSO film was formed to a thickness of 70 nmover the substrate 200. The electrode area of the electrode 101 was setto 4 mm² (2 mm×2 mm).

As the hole-injection layer 111, DBT3P-II and MoO₃ were deposited overthe electrode by co-evaporation such that the deposited layer had aweight ratio of DBT3P-II: MoO₃=1:0.5 and a thickness of 20 nm.

As the hole-transport layer 112, Cz2DBT was deposited over thehole-injection layer by evaporation to a thickness of 20 nm.

As the light-emitting layer 160,bis[2-(diphenylphosphino)phenyl]etheroxide (abbreviation: DPEPO) and4PCCzBfpm were deposited over the hole-transport layer 112 byco-evaporation such that the deposited layer had a weight ratio ofDPEPO: 4PCCzBfpm=0.85:0.15 and a thickness of 15 nm.

As the electron-transport layer 118, DPEPO and1,3,5-tris[3-(3-pyridyl)-phenyl]benzene (abbreviation: TmPyPB) weresuccessively deposited by evaporation to thicknesses of 5 nm and nm,respectively, over the light-emitting layer 160. Then, as theelectron-injection layer 119, LiF was deposited over theelectron-transport layer 118 by evaporation to a thickness of 1 nm. Notethat DPEPO in the electron-transport layer 118 also has a function as anexciton-blocking layer, i.e., prevents excitons generated in thelight-emitting layer 160 from diffusing to the electrode 102 side.

As the electrode 102, aluminum (Al) was formed over theelectron-injection layer 119 to a thickness of 200 nm.

Next, in a glove box containing a nitrogen atmosphere, the comparativelight-emitting element 2 was sealed by fixing the substrate 220 to thesubstrate 200 over which the organic material was deposited using asealant for an organic EL device. For the detailed method, descriptionof the light-emitting element 1 can be referred to. Through the abovesteps, the comparative light-emitting element 2 was obtained.

<Characteristics of Light-Emitting Element>

FIG. 57 shows current efficiency vs. luminance characteristics of thelight-emitting element 5; FIG. 58 shows luminance vs. voltagecharacteristics thereof, FIG. 59 shows external quantum efficiency vs.luminance characteristics thereof, and FIG. 60 shows power efficiencyvs. luminance characteristics thereof. Note that the measurement for thelight-emitting element was performed at room temperature (in anatmosphere kept at 23° C.) by a measurement method similar to that usedin Example 1.

Table 11 shows element characteristics of the light-emitting element 5at around 1000 cd/m².

TABLE 11 External Current CIE Current Power quantum Voltage densityChromaticity Luminance efficiency efficiency efficiency (V) (mA/cm²) (x,y) (cd/m²) (cd/A) (lm/W) (%) Light-emitting 3.00 1.38 (0.196, 0.495) 92366.8 70.0 26.5 element 5

FIG. 61 shows an electroluminescence spectrum of the light-emittingelement 5 when a current at a current density of 2.5 mA/cm² was suppliedto the light-emitting element 5.

As shown in FIG. 57 to FIG. 60 and Table 11, the light-emitting element5 has extremely high current efficiency and extremely high externalquantum efficiency. In addition, the maximum external quantum efficiencyof the light-emitting element 5 is 27.3%, which is an excellent value.

As shown in FIG. 61, the electroluminescence spectrum of thelight-emitting element 5 has a peak at a wavelength of 489 nm and a fullwidth at half maximum of 68 nm, and the light-emitting element 5 emitsblue light. The obtained emission spectrum reveals that light is emittedfrom Ir(mpptz-diBuCNp)₃ as the guest material.

The light-emitting element 5 was driven at an extremely low voltage of3.0 V at around cd/m² and thus exhibited high power efficiency.Furthermore, the light emission start voltage (voltages at the time whenthe luminance exceeds 1 cd/m²) of the light-emitting element 5 was 2.4V. The voltage is lower than a voltage corresponding to the energydifference between the LUMO level and the HOMO level of the guestmaterial Ir(mpptz-diBuCNp)₃, which is described in Example 2. Theresults suggest that emission of the light-emitting element 5 isobtained not by direct recombination of carriers in the guest materialbut by recombination of carriers in the material having a smaller energygap.

<Emission Spectra of Host Materials>

In the fabricated light-emitting element (the light-emitting element 5),4PCCzBfpm was used as the host material. FIG. 62 shows measurementresults of emission spectra of a thin film of 4PCCzBfpm. Note that themeasurement method is similar to that used in Example 1.

As shown in FIG. 62, the wavelengths of peaks (including shoulders) onthe shortest wavelength sides of the emission spectra of 4PCCzBfpm thatindicate fluorescent components and phosphorescent components are 455 nmand 480 nm, respectively. Thus, the singlet excitation energy level andthe triplet excitation energy level calculated from the wavelengths ofthe peaks (including shoulders) are 2.72 eV and 2.58 eV, respectively.That is, the energy difference between the singlet excitation energylevel and the triplet excitation energy level of 4PCCzBfpm calculatedfrom the wavelengths of the peaks (including shoulders) was 0.14 eV,which is extremely small.

Furthermore, as shown in FIG. 62, the wavelengths of the rising portionson the shorter wavelength sides of the emission spectra of 4PCCzBfpmthat indicate fluorescent components and phosphorescent components are435 nm and 464 nm, respectively. Thus, the singlet excitation energylevel and the triplet excitation energy level calculated from thewavelengths of the rising portions are 2.85 eV and 2.67 eV,respectively. That is, the energy difference between the singletexcitation energy level and the triplet excitation energy levelcalculated from the wavelengths of the rising portions of the emissionspectra of 4PCCzBfpm is 0.18 eV, which is also extremely small.

The peak wavelength on the shortest wavelength side of the emissionspectrum of 4PCCzBfpm that indicates phosphorescence components isshorter than or equal to that of the electroluminescence spectra of theguest material (Ir(mpptz-diBuCNp)₃) of the light-emitting element 5.Since Ir(mpptz-diBuCNp)₃ serving as a guest material is a phosphorescentmaterial, light is emitted from the triplet excited state. That is, thetriplet excitation energy of 4PCCzBfpm is higher than the tripletexcitation energy of the guest material.

In addition, as described in Example 2, the absorption band on thelowest energy side (the longest wavelength side) of the absorptionspectrum of Ir(mpptz-diBuCNp)₃ is at around nm and has a regionoverlapping with the fluorescence spectrum of 4PCCzBfpm. Therefore, inthe light-emitting element using 4PCCzBfpm as a host material,excitation energy can be effectively transferred to the guest material.

<Transient Fluorescent Characteristics of Host Material>

Next, transient fluorescent characteristics of 4PCCzBfpm were measuredusing time-resolved emission measurement.

The time-resolved emission measurements were performed on a thin-filmsample in which DPEPO and 4PCCzBfpm were deposited by co-evaporationover a quartz substrate such that the deposited layer had a thickness of50 nm and a weight ratio of DPEPO: 4PCCzBfpm=0.8:0.2. Note that themeasurement method is similar to that used in Example 1.

FIGS. 63A and 63B show transient fluorescent characteristics of4PCCzBfpm obtained by the measurement. FIG. 63A shows measurementresults of emission components having a short emission lifetime, andFIG. 63B shows measurement results of emission components having a longemission lifetime.

The attenuation curves shown in FIGS. 63A and 63B were fitted withFormula 4. The fitting results show that the emission component of thethin film sample of 4PCCzBfpm contains at least a prompt fluorescentcomponent having a fluorescence lifetime of 11.7 μs and a delayedfluorescent component having a fluorescence lifetime of 217 μs which isthe longest. In other words, it is found that 4PCCzBfpm is a thermallyactivated delayed fluorescent material exhibiting delayed fluorescent atroom temperature.

<Characteristics of Comparative Light-Emitting Element>

FIG. 64 shows current efficiency vs. luminance characteristics of thecomparative light-emitting element 2 in which 4PCCzBfpm is used as alight-emitting material; FIG. 65 shows luminance vs. voltagecharacteristics thereof; FIG. 66 shows external quantum efficiency vs.luminance characteristics thereof, and FIG. 67 shows power efficiencyvs. luminance characteristics thereof. Note that the measurement wasperformed at room temperature (in an atmosphere kept at 23° C.).

Table 12 shows the element characteristics of the comparativelight-emitting element 2 at around 100 cd/m².

TABLE 12 External Current CIE Current Power quantum Voltage densityChromaticity Luminance efficiency efficiency efficiency (V) (mA/cm²) (x,y) (cd/m²) (cd/A) (lm/W) (%) Comparative light- 3.7 0.40 (0.17, 0.26) 9323 20 14 emitting element 2

FIG. 68 shows an emission spectrum of the comparative light-emittingelement 2 when a current with a current density of 2.5 mA/cm² wassupplied to the comparative light-emitting element 2.

As shown in FIG. 64 to FIG. 67 and Table 12, the comparativelight-emitting element 2 has high current efficiency and high externalquantum efficiency. The maximum external quantum efficiency of thecomparative light-emitting element 2 is 23.9%, which is an excellentvalue. The reason why the external quantum efficiency of the comparativelight-emitting element 2 is higher than 6.25% is that, as describedabove, 4PCCzBfpm is a material having a small difference between thesinglet excitation energy level and the triplet excitation energy leveland exhibiting thermally activated delayed fluorescence, and has afunction of emitting light originating from singlet excitons generatedby reverse intersystem crossing from triplet excitons as well as lightoriginating from singlet excitons generated by recombination of carriers(holes and electrons) injected from the pair of electrodes.

Meanwhile, as shown in FIG. 68, the peak wavelength of theelectroluminescence spectrum of the comparative light-emitting element 2is 476 nm, which is shorter than the peak wavelengths of theelectroluminescence spectra of the light-emitting element 5. This alsoindicates that the triplet excitation energy level of 4PCCzBfpm ishigher than the triplet excitation energy level of the guest material(Ir(mpptz-diBuCNp)₃) (which is derived from a small energy difference,0.1 eV, between the singlet excitation energy level and the tripletexcitation energy level of 4PCCzBfpm) and 4PCCzBfpm can be suitably usedas the host material of the light-emitting element 5.

<Results of CV Measurement>

The electrochemical characteristics (oxidation reaction characteristicsand reduction reaction characteristics) of 4PCCzBfpm used as the hostmaterial of the light-emitting elements were examined by cyclicvoltammetry (CV). The measurement method was similar to that used inExample 1.

Table 13 shows oxidation potentials and reduction potentials obtained byCV measurement and HOMO levels and LUMO levels of the compoundscalculated from the CV measurement results. Note that Table 13 alsoshows the oxidation potential, the reduction potential, the HOMO level,and the LUMO level of the guest material (Ir(mpptz-diBuCNp)₃) shown inExample 2.

TABLE 13 HOMO level LUMO level calculated from calculated from OxidationReduction oxidation reduction Abbreviation potential (V) potential (V)potential (eV) potential (eV) Ir(mpptz-diBuCNp)₃ 0.46 −2.46 −5.40 −2.494PCCzBfpm 0.76 −2.10 −5.70 −2.84

As shown in Table 13, in the light-emitting element 5, the reductionpotential of the guest material (Ir(mpptz-diBuCNp)₃) is lower than thereduction potential of the host material (4PCCzBfpm), and the oxidationpotential of the guest material (Ir(mpptz-diBuCNp)₃) is lower than theoxidation potential of the host material (4PCCzBfpm). Therefore, theLUMO level of the guest material (Ir(mpptz-diBuCNp)₃) is higher than theLUMO level of the host material (4PCCzBfpm), and the HOMO level of theguest material (Ir(mpptz-diBuCNp)₃) is higher than the HOMO level of thehost material (4PCCzBfpm). The energy difference between the LUMO leveland the HOMO level of the guest material (Ir(mpptz-diBuCNp)₃) is largerthan the energy difference between the LUMO level and the HOMO level ofthe host material (4PCCzBfpm).

Consequently, as described in Example 2, in the guest material of thelight-emitting element 5, the energy difference between the LUMO leveland the HOMO level is greater than the transition energy calculated fromthe absorption edge by 0.3 eV or more. In addition, the energydifference between the LUMO level and the HOMO level is greater than thelight emission energy by 0.4 eV or more. Therefore, high energycorresponding to the energy difference between the LUMO level and theHOMO level is needed, that is, high voltage is needed when carriersinjected from a pair of electrodes are directly recombined in the guestmaterial.

Meanwhile, the energy difference between the LUMO level and the HOMOlevel of the host material (4PCCzBfpm) in the light-emitting element 5was calculated to be 2.86 eV from Table 13. That is, the energydifference between the LUMO level and the HOMO level of the hostmaterial (4PCCzBfpm) of the light-emitting element 5 is smaller than theenergy difference (2.92 eV) between the LUMO level and the HOMO level ofthe guest material (Ir(mpptz-diBuCNp)₃), greater than the transitionenergy (2.59 eV) calculated from the absorption edge, and greater thanthe light emission energy (2.48 eV). Therefore, in the light-emittingelement 5, the guest material can be excited by energy transfer throughan excited state of the host material without the direct carrierrecombination in the guest material, whereby the driving voltage can belowered. Thus, the power consumption of the light-emitting element ofone embodiment of the present invention can be reduced.

According to the CV measurement results in Table 13, among carriers(electrons and holes) injected from the pair of electrodes of thelight-emitting element 5, electrons tend to be injected into the hostmaterial (4PCCzBfpm) with a low LUMO level, whereas holes tend to beinjected into the guest material (Ir(mpptz-diBuCNp)₃) with a high HOMOlevel. That is, there is a possibility that an exciplex is formed by thehost material and the guest material.

The energy difference between the LUMO level of the host material(4PCCzBfpm) and the HOMO level of the guest material(Ir(mpptz-diBuCNp)₃) was calculated from the CV measurement resultsshown in Table 13 and found to be 2.56 eV.

From these results, in the light-emitting element 5, the energydifference (2.56 eV) between the LUMO level of the host material(4PCCzBfpm) and the HOMO level of the guest material(Ir(mpptz-diBuCNp)₃) is greater than or equal to the energy (2.48 eV) oflight emitted by the guest material. Accordingly, rather than formationof an exciplex by the host material and the guest material, transfer ofexcitation energy to the guest material is more facilitated eventually,whereby efficient light emission from the guest material is achieved.This relationship is a feature of one embodiment of the presentinvention for efficient light emission.

In the case where the HOMO level of a guest material is higher than theHOMO level of a host material and the energy difference between the LUMOlevel and the HOMO level of the guest material is larger than the energydifference between the LUMO level and the HOMO level of the hostmaterial as in the light-emitting element 5, a light-emitting elementwith high emission efficiency and low driving voltage can be obtainedwhen the energy difference between the LUMO level and the HOMO level ofthe host material is greater than or equal to the transition energycalculated from the absorption edge of the absorption spectrum of theguest material or the light emission energy of the guest material.Furthermore, in the case where the energy difference between the LUMOlevel and the HOMO level of a guest material is greater than thetransition energy calculated from the absorption edge of the absorptionspectrum of the guest material or greater than or equal to the lightemission energy of the guest material by 0.3 eV or more, alight-emitting element with high emission efficiency and low drivingvoltage can be obtained.

As described above, by employing the structure of one embodiment of thepresent invention, a light-emitting element having high emissionefficiency can be fabricated. Furthermore, a light-emitting element withreduced power consumption can be fabricated, and a light-emittingelement having high emission efficiency and emitting blue light can befabricated.

The structures described in this example can be used in an appropriatecombination with any of the other embodiments and examples.

Example 4

In this example, an example of fabricating a light-emitting element ofan embodiment of the present invention (a light-emitting element 6) isdescribed. Schematic cross-sectional views of the light-emittingelements fabricated in this example are similar to those shown in FIG.37. Table 14 shows details of the element structures. In addition,structures and abbreviations of compounds used here are given below.Note that the above example can be referred to for other compounds.

TABLE 14 Thickness Weight Layer Symbol (nm) Material ratio Light-Electrode 102 200 Al — emitting Electron-injection layer 119 1 LiF —element 6 Electron-transport layer   118(2) 10 BPhen —   118(1) 204PCCzBfpm-02 — Light-emitting layer 160 40 4PCCzBfpm-02:Ir(ppy)₃ 0.9:0.1Hole-transport layer 112 20 mCzFLP — Hole-injection layer 111 60DBT3P-II:MoO₃  1:0.5 Electrode 101 70 ITSO —

<Fabrication of Light-Emitting Element> <<Fabrication of Light-EmittingElement 6>>

As the electrode 101, an ITSO film was formed to a thickness of 70 nmover the substrate 200. The electrode area of the electrode 101 was setto 4 mm² (2 mm×2 mm).

As the hole-injection layer 111, DBT3P-II and MoO₃ were deposited overthe electrode 101 by co-evaporation such that the deposited layer had aweight ratio of DBT3P-II: MoO₃=1:0.5 and a thickness of 60 nm.

As the hole-transport layer 112,9-[3-(9-phenyl-9H-fluoren-9-yl)phenyl]-9H-carbazole (abbreviation:mCzFLP) was deposited over the hole-injection layer 111 by evaporationto a thickness of 20 nm.

As the light-emitting layer 160,4-(9′-phenyl-2,3′-bi-9H-carbazol-9-yl)benzofuro[3,2-d]pyrimidine(abbreviation: 4PCCzBfpm-02) and Ir(ppy)₃ were deposited over thehole-transport layer 112 by co-evaporation such that the deposited layerhad a weight ratio of 4PCCzBfpm-02: Ir(ppy)₃=0.9:0.1 and a thickness of40 nm. Note that in the light-emitting layer 160, Ir(ppy)₃ correspondsto a guest material and 4PCCzBfpm-02 corresponds to a host material.

As the electron-transport layer 118, 4PCCzBfpm-02 and BPhen weresuccessively deposited by evaporation to thicknesses of 20 nm and 10 nm,respectively, over the light-emitting layer 160. As theelectron-injection layer 119, lithium fluoride (LiF) was deposited overthe electron-transport layer 118 by evaporation to a thickness of 1 nm.

As the electrode 102, aluminum (Al) was formed over theelectron-injection layer 119 to a thickness of 200 nm.

Next, in a glove box containing a nitrogen atmosphere, thelight-emitting element 6 was sealed by fixing the substrate 220 to thesubstrate 200 over which the organic material was deposited using asealant for an organic EL device. For the detailed method, descriptionof the light-emitting element 1 can be referred to. Through the abovesteps, the light-emitting element 6 was obtained.

<Characteristics of Light-Emitting Element>

FIG. 69 shows current efficiency vs. luminance characteristics of thelight-emitting element 6; FIG. 70 shows luminance vs. voltagecharacteristics thereof, FIG. 71 shows external quantum efficiency vs.luminance characteristics thereof, and FIG. 72 shows power efficiencyvs. luminance characteristics thereof. Note that the measurement for thelight-emitting element was performed at room temperature (in anatmosphere kept at 23° C.) by a measurement method similar to that usedin Example 1.

Table 15 shows element characteristics of the light-emitting element 6at around 1000 cd/m².

TABLE 15 External Current CIE Current Power quantum Voltage densityChromaticity Luminance efficiency efficiency efficiency (V) (mA/cm²) (x,y) (cd/m²) (cd/A) (lm/W) (%) Light-emitting 4.40 1.98 (0.347, 0.616)1220 61.6 44.0 17.2 element 6

FIG. 73 shows an electroluminescence spectrum of the light-emittingelement 6 when a current at a current density of 2.5 mA/cm² was suppliedto the light-emitting element 6.

As shown in FIG. 69 to FIG. 72 and Table 15, the light-emitting element6 has extremely high current efficiency and extremely high externalquantum efficiency. In addition, the maximum external quantum efficiencyof the light-emitting element 6 is 17.7%, which is an excellent value.

As shown in FIG. 73, the electroluminescence spectrum of thelight-emitting element 6 has a peak at a wavelength of 519 nm and a fullwidth at half maximum of 83 nm, and the light-emitting element 6 emitsgreenlight. The obtained emission spectrum reveals that light is emittedfrom Ir(ppy)₃ as the guest material.

The light-emitting element 6 was driven at a low voltage of 4.4 V ataround 1000 cd/m² and thus exhibited high power efficiency. Furthermore,the light emission start voltage (voltages at the time when theluminance exceeds 1 cd/m²) of the light-emitting element 6 was 2.7 V.The voltage is lower than a voltage corresponding to the energydifference between the LUMO level and the HOMO level of the guestmaterial Ir(ppy)₃, which is described later. The results suggest thatemission of the light-emitting element 6 is obtained not by directrecombination of carriers in the guest material but by recombination ofcarriers in the material having a smaller energy gap.

<Emission Spectra of Host Materials>

In the fabricated light-emitting element (the light-emitting element 6),4PCCzBfpm-02 was used as the host material. FIG. 74 shows measurementresults of emission spectra of a thin film of 4PCCzBfpm-02. Note thatthe measurement method is similar to that used in Example 1.

As shown in FIG. 74, the wavelengths of peaks (including shoulders) onthe shortest wavelength sides of the emission spectra of 4PCCzBfpm-02that indicate fluorescent components and phosphorescent components are458 nm and 495 nm, respectively. Thus, the singlet excitation energylevel and the triplet excitation energy level calculated from thewavelengths of the peaks (including shoulders) are 2.71 eV and 2.51 eV,respectively. That is, the energy difference between the singletexcitation energy level and the triplet excitation energy level of4PCCzBfpm-02 calculated from the wavelengths of the peaks (includingshoulders) was 0.20 eV, which is extremely small.

The peak wavelength on the shortest wavelength side of the emissionspectrum of 4PCCzBfpm-02 that indicates phosphorescence components isshorter than or equal to that of the electroluminescence spectra of theguest material (Ir(ppy)₃) of the light-emitting element 6. SinceIr(ppy)₃ serving as a guest material is a phosphorescent material, lightis emitted from the triplet excited state. That is, the tripletexcitation energy of 4PCCzBfpm-02 is higher than the triplet excitationenergy of the guest material.

<Absorption Spectrum and Emission Spectrum of Guest Material>

FIG. 75 shows the measurement results of the absorption spectrum andemission spectrum of Ir(ppy)₃ that is the guest material in thelight-emitting element. Note that the measurement method is similar tothat used in Example 1.

As shown in FIG. 75, the absorption band on the lowest energy side (thelongest wavelength side) of the absorption spectrum of Ir(ppy)₃ is ataround 500 nm. The absorption edge was obtained from data of theabsorption spectrum, and the transition energy was estimated on theassumption of direct transition. As a result, the absorption edge ofIr(ppy)₃ was 508 nm and the transition energy was calculated to be 2.44eV.

As described above, the absorption band on the lowest energy side (thelongest wavelength side) of the absorption spectrum of Ir(ppy)₃ is ataround 500 nm and has a region overlapping with the fluorescentcomponent of the emission spectrum of 4PCCzBfpm-02. Therefore, in thelight-emitting element using 4PCCzBfpm-02 as a host material, excitationenergy can be effectively transferred to the guest material. Thissuggests that 4PCCzBfpm-02 is suitably used as a host material of thelight-emitting element 6.

<Results of CV Measurement>

The electrochemical characteristics (oxidation reaction characteristicsand reduction reaction characteristics) of the compounds used as theguest material and the host material of the light-emitting element wereexamined by cyclic voltammetry (CV). The measurement method was similarto that used in Example 1.

Table 16 shows oxidation potentials and reduction potentials obtained byCV measurement and HOMO levels and LUMO levels of the compoundscalculated from the CV measurement results.

TABLE 16 HOMO level LUMO level calculated from calculated from OxidationReduction oxidation reduction Abbreviation potential (V) potential (V)potential (eV) potential (eV) Ir(ppy)3 0.38 −2.63 −5.32 −2.314PCCzBfpm-02 0.82 −2.10 −5.76 −2.84

As shown in Table 16, in the light-emitting element 6, the reductionpotential of the guest material (Ir(ppy)₃) is lower than the reductionpotential of the host material (4PCCzBfpm-02), and the oxidationpotential of the guest material (Ir(ppy)₃) is lower than the oxidationpotential of the host material (4PCCzBfpm-02). Therefore, the LUMO levelof the guest material (Ir(ppy)₃) is higher than the LUMO level of thehost material (4PCCzBfpm-02), and the HOMO level of the guest material(Ir(ppy)₃) is higher than the HOMO level of the host material(4PCCzBfpm-02). The energy difference between the LUMO level and theHOMO level of the guest material (Ir(ppy)₃) is larger than the energydifference between the LUMO level and the HOMO level of the hostmaterial (4PCCzBfpm-02).

The energy difference between the LUMO level and the HOMO level ofIr(ppy)₃ was 3.01 eV. The value was calculated from the CV measurementresults shown in Table 16.

As described above, the transition energy of Ir(ppy)₃ calculated fromthe absorption edge of the absorption spectrum of Ir(ppy)₃ is 2.44 eV,and the energy difference between the LUMO level and the HOMO level ofIr(ppy)₃ is larger than the transition energy calculated from theabsorption edge by 0.57 eV.

The peak wavelength on the shortest wavelength side of the emissionspectrum of Ir(ppy)₃ shown in FIG. 75 was 518 nm. According to that, thelight emission energy of Ir(ppy)₃ was calculated to be 2.39 eV.

That is, the energy difference between the LUMO level and the HOMO levelof Ir(ppy)₃ was larger than the light emission energy by 0.62 eV.

Consequently, in the guest material of the light-emitting element, theenergy difference between the LUMO level and the HOMO level is greaterthan the transition energy calculated from the absorption edge by 0.4 eVor more. In addition, the energy difference between the LUMO level andthe HOMO level is greater than the light emission energy by 0.4 eV ormore. Therefore, high energy corresponding to the energy differencebetween the LUMO level and the HOMO level is needed, that is, highvoltage is needed when carriers injected from a pair of electrodes aredirectly recombined in the guest material.

Meanwhile, the energy difference between the LUMO level and the HOMOlevel of the host material (4PCCzBfpm-02) in the light-emitting element6 was calculated to be 2.92 eV from Table 16. That is, the energydifference between the LUMO level and the HOMO level of the hostmaterial (4PCCzBfpm-02) of the light-emitting element 6 is smaller thanthe energy difference (3.01 eV) between the LUMO level and the HOMOlevel of the guest material (Ir(ppy)₃), greater than the transitionenergy (2.44 eV) calculated from the absorption edge, and greater thanthe light emission energy (2.39 eV). Therefore, in the light-emittingelement 6, the guest material can be excited by energy transfer throughan excited state of the host material without the direct carrierrecombination in the guest material, whereby the driving voltage can belowered. Thus, the power consumption of the light-emitting element ofone embodiment of the present invention can be reduced.

According to the CV measurement results in Table 16, among carriers(electrons and holes) injected from the pair of electrodes of thelight-emitting element 6, electrons tend to be injected into the hostmaterial (4PCCzBfpm-02) with a low LUMO level, whereas holes tend to beinjected into the guest material (Ir(ppy)₃) with a high HOMO level. Thatis, there is a possibility that an exciplex is formed by the hostmaterial and the guest material.

The energy difference between the LUMO level of the host material(4PCCzBfpm-02) and the HOMO level of the guest material (Ir(ppy)₃) wascalculated from the CV measurement results shown in Table 16 and foundto be 2.48 eV.

From these results, in the light-emitting element 6, the energydifference (2.48 eV) between the LUMO level of the host material(4PCCzBfpm-02) and the HOMO level of the guest material (Ir(ppy)₃) isgreater than or equal to the energy (2.39 eV) of light emitted by theguest material. Accordingly, rather than formation of an exciplex by thehost material and the guest material, transfer of excitation energy tothe guest material is more facilitated eventually, whereby efficientlight emission from the guest material is achieved. This relationship isa feature of one embodiment of the present invention for efficient lightemission.

In the case where the HOMO level of a guest material is higher than theHOMO level of a host material and the energy difference between the LUMOlevel and the HOMO level of the guest material is larger than the energydifference between the LUMO level and the HOMO level of the hostmaterial as in the light-emitting element 6, a light-emitting elementwith high emission efficiency and low driving voltage can be obtainedwhen the energy difference between the LUMO level and the HOMO level ofthe host material is greater than or equal to the transition energycalculated from the absorption edge of the absorption spectrum of theguest material or the light emission energy of the guest material.Furthermore, in the case where the energy difference between the LUMOlevel and the HOMO level of a guest material is greater than thetransition energy calculated from the absorption edge of the absorptionspectrum of the guest material or greater than or equal to the lightemission energy of the guest material by 0.4 eV or more, alight-emitting element with high emission efficiency and low drivingvoltage can be obtained.

As described above, by employing the structure of one embodiment of thepresent invention, a light-emitting element having high emissionefficiency can be fabricated. Furthermore, a light-emitting element withreduced power consumption can be fabricated, and a light-emittingelement having high emission efficiency and emitting green light can befabricated.

The structures described in this example can be used in an appropriatecombination with any of the other embodiments and examples.

Example 5

In this example, an example of fabricating a light-emitting element ofan embodiment of the present invention (a light-emitting element 7) isdescribed. Schematic cross-sectional views of the light-emittingelements fabricated in this example are similar to those shown in FIG.37. Table 17 shows details of the element structures. In addition,structures and abbreviations of compounds used

TABLE 17 Thickness Weight Layer Symbol (nm) Material ratioLight-emitting Electrode 102 200 Al — element 7 Electron-injection layer119 1 LiF — Electron-transport layer   118(2) 15 BPhen —   118(1) 104mPCCzPBfpm-02 — Light-emitting layer 160 40 4mPCCzPBfpm-02:Ir(ppy)₃0.9:0.1 Hole-transport layer 112 20 mCzFLP — Hole-injection layer 111 15DBT3P-II:MoO₃  1:0.5 Electrode 101 70 ITSO —

<Fabrication of Light-Emitting Element> <<Fabrication of Light-EmittingElement 7>>

As the electrode 101, an ITSO film was formed to a thickness of 70 nmover the substrate 200. The electrode area of the electrode 101 was setto 4 mm² (2 mm×2 mm).

As the hole-injection layer 111, DBT3P-II and MoO₃ were deposited overthe electrode 101 by co-evaporation such that the deposited layer had aweight ratio of DBT3P-II: MoO₃=1:0.5 and a thickness of 60 nm.

As the hole-transport layer 112, mCzFLP was deposited over thehole-injection layer 111 by evaporation to a thickness of 20 nm.

As the light-emitting layer 160,4-[3-(9′-phenyl-2,3′-bi-9H-carbazol-9-yl)phenyl]benzofuro[3,2-d]pyrimidine(abbreviation: 4mPCCzPBfpm-02) and Ir(ppy)₃ were deposited over thehole-transport layer 112 by co-evaporation such that the deposited layerhad a weight ratio of 4mPCCzPBfpm-02: Ir(ppy)₃=0.9:0.1 and a thicknessof 40 nm. Note that in the light-emitting layer 160, Ir(ppy)₃corresponds to a guest material and 4mPCCzPBfpm-02 corresponds to a hostmaterial.

As the electron-transport layer 118, 4mPCCzPBfpm-02 and BPhen weresuccessively deposited by evaporation to thicknesses of 20 nm and 10 nm,respectively, over the light-emitting layer 160. As theelectron-injection layer 119, lithium fluoride (LiF) was deposited overthe electron-transport layer 118 by evaporation to a thickness of 1 nm.

As the electrode 102, aluminum (Al) was formed over theelectron-injection layer 119 to a thickness of 200 nm.

Next, in a glove box containing a nitrogen atmosphere, thelight-emitting element 7 was sealed by fixing the substrate 220 to thesubstrate 200 over which the organic material was deposited using asealant for an organic EL device. For the detailed method, Example 1 canbe referred to. Through the above steps, the light-emitting element 7was obtained.

<Characteristics of Light-Emitting Element>

FIG. 76 shows current efficiency vs. luminance characteristics of thelight-emitting element 7; FIG. 77 shows luminance vs. voltagecharacteristics thereof, FIG. 78 shows external quantum efficiency vs.luminance characteristics thereof, and FIG. 79 shows power efficiencyvs. luminance characteristics thereof. Note that the measurement for thelight-emitting element was performed at room temperature (in anatmosphere kept at 23° C.) by a measurement method similar to that usedin Example 1.

Table 18 shows element characteristics of the light-emitting element 7at around 1000 cd/m².

TABLE 18 External Current CIE Current Power quantum Voltage densityChromaticity Luminance efficiency efficiency efficiency (V) (mA/cm²) (x,y) (cd/m²) (cd/A) (lm/W) (%) Light-emitting 4.00 1.19 (0.381, 0.590) 75563.5 49.9 18.3 element 7

FIG. 80 shows an electroluminescence spectrum of the light-emittingelement 7 when a current at a current density of 2.5 mA/cm² was suppliedto the light-emitting element 7.

As shown in FIG. 76 to FIG. 79 and Table 18, the light-emitting element7 has extremely high current efficiency and extremely high externalquantum efficiency. In addition, the maximum external quantum efficiencyof the light-emitting element 7 is 18.4%, which is an excellent value.

As shown in FIG. 80, the electroluminescence spectrum of thelight-emitting element 7 has a peak at a wavelength of 549 nm and a fullwidth at half maximum of 96 nm, and the light-emitting element 7 emitsgreenlight. The obtained emission spectrum reveals that light is emittedfrom Ir(ppy)₃ as the guest material.

The light-emitting element 7 was driven at a low voltage of 4.0 V ataround 1000 cd/m² and thus exhibited high power efficiency. Furthermore,the light emission start voltage (voltages at the time when theluminance exceeds 1 cd/m²) of the light-emitting element 7 was 2.5 V.The voltage is lower than a voltage corresponding to the energydifference between the LUMO level and the HOMO level of the guestmaterial Ir(ppy)₃, which is described in Example 4. The results suggestthat emission of the light-emitting element 7 is obtained not by directrecombination of carriers in the guest material but by recombination ofcarriers in the material having a smaller energy gap.

<Emission Spectra of Host Materials>

In the fabricated light-emitting element (the light-emitting element 7),4mPCCzPBfpm-02 was used as the host material. FIG. 81 shows measurementresults of emission spectra of a thin film of 4mPCCzPBfpm-02. Note thatthe measurement method is similar to that used in Example 1.

As shown in FIG. 81, the wavelengths of peaks (including shoulders) onthe shortest wavelength sides of the emission spectra of 4mPCCzPBfpm-02that indicate fluorescent components and phosphorescent components are470 nm and 495 nm, respectively. Thus, the singlet excitation energylevel and the triplet excitation energy level calculated from thewavelengths of the peaks (including shoulders) are 2.64 eV and 2.50 eV,respectively. That is, the energy difference between the singletexcitation energy level and the triplet excitation energy level of4mPCCzPBfpm-02 calculated from the wavelengths of the peaks (includingshoulders) was 0.14 eV, which is extremely small.

As described in Example 4, the absorption band on the lowest energy side(the longest wavelength side) of the absorption spectrum of Ir(ppy)₃ isat around 500 nm and has a region overlapping with the fluorescentcomponent of the emission spectrum of 4mPCCzPBfpm-02. Therefore, in thelight-emitting element using 4mPCCzPBfpm-02 as a host material,excitation energy can be effectively transferred to the guest material.This suggests that 4mPCCzPBfpm-02 is suitably used as a host material ofthe light-emitting element 7.

<Results of CV Measurement>

The electrochemical characteristics (oxidation reaction characteristicsand reduction reaction characteristics) of the compounds used as theguest material and the host material of the light-emitting element wereexamined by cyclic voltammetry (CV). The measurement method was similarto that used in Example 1.

Table 19 shows oxidation potentials and reduction potentials obtained byCV measurement and HOMO levels and LUMO levels of the compoundscalculated from the CV measurement results.

TABLE 19 HOMO level LUMO level calculated from calculated from OxidationReduction oxidation reduction Abbreviation potential (V) potential (V)potential (eV) potential (eV) Ir(ppy)₃ 0.38 −2.63 −5.32 −2.314mPCCzPBfpm-02 0.74 −1.92 −5.68 −3.02

As shown in Table 19, in the light-emitting element 7, the reductionpotential of the guest material (Ir(ppy)₃) is lower than the reductionpotential of the host material (4mPCCzPBfpm-02), and the oxidationpotential of the guest material (Ir(ppy)₃) is lower than the oxidationpotential of the host material (4mPCCzPBfpm-02). Therefore, the LUMOlevel of the guest material (Ir(ppy)₃) is higher than the LUMO level ofthe host material (4mPCCzPBfpm-02), and the HOMO level of the guestmaterial (Ir(ppy)₃) is higher than the HOMO level of the host material(4mPCCzPBfpm-02). The energy difference between the LUMO level and theHOMO level of the guest material (Ir(ppy)₃) is larger than the energydifference between the LUMO level and the HOMO level of the hostmaterial (4mPCCzPBfpm-02).

The energy difference between the LUMO level and the HOMO level ofIr(ppy)₃ was 3.01 eV. The value was calculated from the CV measurementresults shown in Table 19.

As described above, the transition energy of Ir(ppy)₃ calculated fromthe absorption edge of the absorption spectrum of Ir(ppy)₃ is 2.44 eV,and the energy difference between the LUMO level and the HOMO level ofIr(ppy)₃ is larger than the transition energy calculated from theabsorption edge by 0.57 eV.

The peak wavelength on the shortest wavelength side of the emissionspectrum of Ir(ppy)₃ shown in FIG. 75 was 518 nm. According to that, thelight emission energy of Ir(ppy)₃ was calculated to be 2.39 eV.

That is, the energy difference between the LUMO level and the HOMO levelof Ir(ppy)₃ was larger than the light emission energy by 0.62 eV.

Consequently, as described in Example 4, in the guest material(Ir(ppy)₃) used in the light-emitting element 7, the energy differencebetween the LUMO level and the HOMO level is greater than the transitionenergy calculated from the absorption edge by 0.4 eV or more. Inaddition, the energy difference between the LUMO level and the HOMOlevel is greater than the light emission energy by 0.4 eV or more.Therefore, high energy corresponding to the energy difference betweenthe LUMO level and the HOMO level is needed, that is, high voltage isneeded when carriers injected from a pair of electrodes are directlyrecombined in the guest material.

Meanwhile, the energy difference between the LUMO level and the HOMOlevel of the host material (4mPCCzPBfpm-02) in the light-emittingelement 7 was calculated to be 2.66 eV from Table 19. That is, theenergy difference between the LUMO level and the HOMO level of the hostmaterial (4mPCCzPBfpm-02) of the light-emitting element 7 is smallerthan the energy difference (3.01 eV) between the LUMO level and the HOMOlevel of the guest material (Ir(ppy)₃), greater than the transitionenergy (2.44 eV) calculated from the absorption edge, and greater thanthe light emission energy (2.39 eV). Therefore, in the light-emittingelement 7, the guest material can be excited by energy transfer throughan excited state of the host material without the direct carrierrecombination in the guest material, whereby the driving voltage can belowered. Thus, the power consumption of the light-emitting element ofone embodiment of the present invention can be reduced.

As described above, by employing the structure of one embodiment of thepresent invention, a light-emitting element having high emissionefficiency can be fabricated. Furthermore, a light-emitting element withreduced power consumption can be fabricated, and a light-emittingelement having high emission efficiency and emitting green light can befabricated.

The structures described in this example can be used in an appropriatecombination with any of the other embodiments and examples.

Reference Example 1

In this reference example, a method for synthesizingtris{2-[4-(4-cyano-2,6-diisobutylphenyl)-5-(2-methylphenyl)-4H-1,2,4-triazol-3-yl-κN²]phenyl-κC}iridium(III)(abbreviation: Ir(mpptz-diBuCNp)₃), which is the organometallic complexused as the guest material in Examples 2 and 3, is described.

Synthesis Example 1 Step 1: Synthesis of4-amino-3,5-diisobutylbenzonitrile

Into a 1000 mL three-neck flask were put 9.4 g (50 mmol) of4-amino-3,5-dichlorobenzonitrile, 26 g (253 mmol) of isobutylboronicacid, 54 g (253 mmol) of tripotassium phosphate, 2.0 g (4.8 mmol) of2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl (S-phos), and 500 mL oftoluene. The air in the flask was replaced with nitrogen, and thismixture was degassed while being stirred under reduced pressure. Afterthe degassing, 0.88 g (0.96 mmol) oftris(dibenzylideneacetone)palladium(0) was added, and the mixture wasstirred at 130° C. under a nitrogen stream for 8 hours to be reacted.Toluene was added to the reacted solution, and the solution was filteredthrough a filter aid in which Celite, aluminum oxide, and Celite werestacked in this order. The obtained filtrate was concentrated to give anoily substance. The obtained oily substance was purified by silica gelcolumn chromatography. Toluene was used as a developing solvent. Theresulting fraction was concentrated to give 10 g of a yellow oilysubstance in a yield of 87%. The obtained yellow oily substance wasidentified as 4-amino-3,5-diisobutylbenzonitrile by nuclear magneticresonance (NMR) spectroscopy. The synthesis scheme of Step 1 is shown in(a-1) below.

Step 2: Synthesis of Hmpptz-diBuCNp

Into a 300 mL three-neck flask were put 11 g (48 mmol) of4-amino-3,5-diisobutylbenzonitrile synthesized in Step 1, 4.7 g (16mmol) ofN-(2-methylphenyl)chloromethylidene-N′-phenylchloromethylidenehydrazine,and 40 mL of N,N-dimethylaniline, and the mixture was stirred at 160° C.under a nitrogen stream for 7 hours to be reacted. After the reaction,the reacted solution was added to 300 mL of 1M hydrochloric acid andstirring was performed for 3 hours. Ethyl acetate was added to thismixture, an organic layer and an aqueous layer were separated and theaqueous layer was subjected to extraction with ethyl acetate. Theorganic layer and the extracted solution were combined, and washed witha saturated aqueous solution of sodium hydrogen carbonate and then withsaturated brine, and anhydrous magnesium sulfate was added to theorganic layer for drying. The obtained mixture was subjected to gravityfiltration, and the filtrate was concentrated to give an oily substance.The obtained oily substance was purified by silica gel columnchromatography. As a developing solvent, a 5:1 hexane-ethyl acetatemixed solvent was used. The obtained fraction was concentrated to give asolid. Hexane was added to the obtained solid, and the mixture wasirradiated with ultrasonic waves and then subjected to suctionfiltration to give 2.0 g of a white solid in a yield of 28%. Theobtained white solid was identified as4-(4-cyano-2,6-diisobutylphenyl)-3-(2-methylphenyl)-5-phenyl-4H-1,2,4-triazole(abbreviation: Hmpptz-diBuCNp) by nuclear magnetic resonance (NMR)spectroscopy. The synthesis scheme of Step 2 is shown in (b-1) below.

Step 3: Synthesis of Ir(mpptz-diBuCNp)₃

Into a reaction container equipped with a three-way cock were put 2.0 g(4.5 mmol) of Hmpptz-diBuCNp synthesized in Step 2 and 0.44 g (0.89mmol) of tris(acetylacetonato)iridium(III), and the mixture was stirredat 250° C. under an argon stream for 43 hours to be reacted. Theobtained reaction mixture was added to dichloromethane, and an insolublematter was removed. The obtained filtrate was concentrated to give asolid. The obtained solid was purified by silica gel columnchromatography. As a developing solvent, dichloromethane was used. Theobtained fraction was concentrated to give a solid. The obtained solidwas recrystallized from ethyl acetate/hexane, so that 0.32 g of a yellowsolid was obtained in a yield of 23%. Then 0.31 g of the obtained yellowsolid was purified by a train sublimation method. The purification bysublimation was performed by heating at 310° C. under a pressure of 2.6Pa with an argon flow rate of 5.0 mL/min for 19 hours. After thepurification by sublimation, 0.26 g of a yellow solid was obtained at acollection rate of 84%. The synthesis scheme of Step 3 is shown in (c-1)below.

The protons (¹H) of the yellow solid that was obtained in Step 3 weremeasured by nuclear magnetic resonance (NMR) spectroscopy.

¹H-NMR δ(CDCl₃): 0.33 (d, 18H), 0.92 (d, 18H), 1.51-1.58 (m, 3H),1.80-1.88 (m, 6H), 2.10-2.15 (m, 6H), 2.26-2.30 (m, 3H), 2.55 (s, 9H),6.12 (d, 3H), 6.52 (t, 3H), 6.56 (d, 3H), 6.72 (t, 3H), 6.83 (t, 3H),6.97 (d, 3H), 7.16 (t, 3H), 7.23 (d, 3H), 7.38 (s, 3H), 7.55 (s, 3H).

Reference Example 2

In this reference example, a method for synthesizing4-(9′-phenyl-3,3′-bi-9H-carbazol-9-yl)benzofuro[3,2-d]pyrimidine(abbreviation: 4PCCzBfpm), which is the compound used as the hostmaterial in Example 3, is described.

Synthesis Example 2 <<Synthesis of 4PCCzBfpm>>

First, 0.15 g (3.6 mmol) of sodium hydride (60%) was put into athree-neck flask the air in which was replaced with nitrogen, and 10 mLof N,N-dimethylformamide (abbreviation: DMF) was dropped thereinto whilestirring was performed. The container was cooled down to 0° C., a mixedsolution of 1.1 g (2.7 mmol) of 9-phenyl-3,3′-bi-9H-carbazole and 15 mLof DMF was dropped thereinto, and stirring was performed at roomtemperature for 30 minutes. Then, the container was cooled down to 0°C., a mixed solution of 0.50 g (2.4 mmol) of4-chloro[1]benzofuro[3,2-d]pyrimidine and 15 mL of DMF was added, andstirring was performed at room temperature for 20 hours. The resultingreaction solution was put into ice water and toluene was added to themixture. An organic layer was extracted from the resulting mixture withthe use of ethyl acetate and washed with saturated brine. Magnesiumsulfate was added and filtration was performed. The solvent of theobtained filtrate was distilled off and purification was conducted bysilica gel column chromatography (developing solvent: toluene, and thena mixed solvent of toluene:ethyl acetate=1:20). Recrystallization usinga mixed solvent of toluene and hexane was performed, so that 1.0 g of4PCCzBfpm, which was the target substance, was obtained as a yellowishwhite solid in a yield of 72%. Then, 1.0 g of the yellowish white solidwas purified using a train sublimation method. In the purification bysublimation, the yellowish white solid was heated at 270° C. to 280° C.with the pressure set at 2.6 Pa and the argon gas flow rate set at 5mL/min. After the purification by sublimation, 0.7 g of a yellowishwhite solid, which was the target substance, was obtained at acollection rate of 69%. The synthesis scheme of this step is shown in(A-2) below.

Analysis results by nuclear magnetic resonance (H-NMR) spectroscopy ofthe yellowish white solid obtained in the above step are describedbelow. These results reveal that 4PCCzBfpm was obtained.

¹H-NMR δ(CDC₃): 7.31-7.34 (m, 1H), 7.43-7.46 (m, 3H), 7.48-7.54 (m, 3H),7.57-7.60 (t, 1H), 7.62-7.66 (m, 4H), 7.70 (d, 1H), 7.74-7.77 (dt, 1H),7.80 (dd, 1H), 7.85 (dd, 1H), 7.88-7.93 (m, 2H), 8.25 (d, 2H), 8.37 (d,1H), 8.45 (ds, 1H), 8.49 (ds, 1H), 9.30 (s, 1H).

EXPLANATION OF REFERENCE

100: EL layer, 101: electrode, 101 a: conductive layer, 101 b:conductive layer, 101 c: conductive layer, 102: electrode, 103:electrode, 103 a: conductive layer, 103 b: conductive layer, 104:electrode, 104 a: conductive layer, 104 b: conductive layer, 106:light-emitting unit, 108: light-emitting unit, 110: light-emitting unit,111: hole-injection layer, 112: hole-transport layer, 113:electron-transport layer, 114: electron-injection layer, 115:charge-generation layer, 116: hole-injection layer, 117: hole-transportlayer, 118: electron-transport layer, 119: electron-injection layer,120: light-emitting layer, 121: guest material, 122: host material,123B: light-emitting layer, 123G: light-emitting layer, 123R:light-emitting layer, 130: light-emitting layer, 131: guest material,132: host material, 133: host material, 135: light-emitting layer, 140:light-emitting layer, 141: guest material, 142: host material, 142_1:organic compound, 142_2: organic compound, 145: partition wall, 150:light-emitting element, 152: light-emitting element, 160: light-emittinglayer, 170: light-emitting layer, 190: light-emitting layer, 190 a:light-emitting layer, 190 b: light-emitting layer, 200: substrate, 220:substrate, 221B: region, 221G: region, 221R: region, 222B: region, 222G:region, 222R: region, 223: light-blocking layer, 224B: optical element,224G: optical element, 224R: optical element, 250: light-emittingelement, 252: light-emitting element, 260 a: light-emitting element, 260b: light-emitting element, 262 a: light-emitting element, 262 b:light-emitting element, 301_1: wiring, 301_5: wiring, 301_6: wiring,301_7: wiring, 302_1: wiring, 302_2: wiring, 303_1: transistor, 303_6:transistor, 303_7: transistor, 304: capacitor, 304_1: capacitor, 304_2:capacitor, 305: light-emitting element, 306_1: wiring, 306_3: wiring,307_1: wiring, 307_3: wiring, 308_1: transistor, 308_6: transistor,309_1: transistor, 309_2: transistor, 311_1: wiring, 311_3: wiring,312_1: wiring, 312_2: wiring, 600: display device, 601: signal linedriver circuit portion, 602: pixel portion, 603: scan line drivercircuit portion, 604: sealing substrate, 605: sealing material, 607:region, 607 a: sealing layer, 607 b: sealing layer, 607 c: sealinglayer, 608: wiring, 609: FPC, 610: element substrate, 611: transistor,612: transistor, 613: lower electrode, 614: partition wall, 616: ELlayer, 617: upper electrode, 618: light-emitting element, 621: opticalelement, 622: light-blocking layer, 623: transistor, 624: transistor,801: pixel circuit, 802: pixel portion, 804: driver circuit portion, 804a: scan line driver circuit, 804 b: signal line driver circuit, 806:protection circuit, 807: terminal portion, 852: transistor, 854:transistor, 862: capacitor, 872: light-emitting element, 1001:substrate, 1002: base insulating film, 1003: gate insulating film, 1006:gate electrode, 1007: gate electrode, 1008: gate electrode, 1020:interlayer insulating film, 1021: interlayer insulating film, 1022:electrode, 1024B: lower electrode, 1024G: lower electrode, 1024R: lowerelectrode, 1024Y: lower electrode, 1025: partition wall, 1026: upperelectrode, 1028: EL layer, 1028B: light-emitting layer, 1028G:light-emitting layer, 1028R: light-emitting layer, 1028Y: light-emittinglayer, 1029: sealing layer, 1031: sealing substrate, 1032: sealingmaterial, 1033: base material, 1034B: coloring layer, 1034G: coloringlayer, 1034R: coloring layer, 1034Y: coloring layer, 1035:light-blocking layer, 1036: overcoat layer, 1037: interlayer insulatingfilm, 1040: pixel portion, 1041: driver circuit portion, 1042:peripheral portion, 2000: touch panel, 2001: touch panel, 2501: displaydevice, 2502R: pixel, 2502 t: transistor, 2503 c: capacitor, 2503 g:scan line driver circuit, 2503 s: signal line driver circuit, 2503 t:transistor, 2509: FPC, 2510: substrate, 2510 a: insulating layer, 2510b: flexible substrate, 2510 c: adhesive layer, 2511: wiring, 2519:terminal, 2521: insulating layer, 2528: partition wall, 2550R:light-emitting element, 2560: sealing layer, 2567BM: light-blockinglayer, 2567 p: anti-reflective layer, 2567R: coloring layer, 2570:substrate, 2570 a: insulating layer, 2570 b: flexible substrate, 2570 c:adhesive layer, 2580R: light-emitting module, 2590: substrate, 2591:electrode, 2592: electrode, 2593: insulating layer, 2594: wiring, 2595:touch sensor, 2597: adhesive layer, 2598: wiring, 2599: connectionlayer, 2601: pulse voltage output circuit, 2602: current sensingcircuit, 2603: capacitance, 2611: transistor, 2612: transistor, 2613:transistor, 2621: electrode, 2622: electrode, 3000: light-emittingdevice, 3001: substrate, 3003: substrate, 3005: light-emitting element,3007: sealing region, 3009: sealing region, 3011: region, 3013: region,3014: region, 3015: substrate, 3016: substrate, 3018: desiccant, 3054:display portion, 3500: multifunction terminal, 3502: housing, 3504:display portion, 3506: camera, 3508: lighting, 3600: light, 3602:housing, 3608: lighting, 3610: speaker, 7101: housing, 7102: housing,7103: display portion, 7104: display portion, 7105: microphone, 7106:speaker, 7107: operation key, 7108: stylus, 7121: housing, 7122: displayportion, 7123: keyboard, 7124: pointing device, 7200: head-mounteddisplay, 7201: mounting portion, 7202: lens, 7203: main body, 7204:display portion, 7205: cable, 7206: battery, 7300: camera, 7301:housing, 7302: display portion, 7303: operation button, 7304: shutterbutton, 7305: connection portion, 7306: lens, 7400: finder, 7401:housing, 7402: display portion, 7403: button, 7701: housing, 7702:housing, 7703: display portion, 7704: operation key, 7705: lens, 7706:joint, 8000: display module, 8001: upper cover, 8002: lower cover, 8003:FPC, 8004: touch sensor, 8005: FPC, 8006: display device, 8009: frame,8010: printed board, 8011: battery, 8501: lighting device, 8502:lighting device, 8503: lighting device, 8504: lighting device, 9000:housing, 9001: display portion, 9003: speaker, 9005: operation key,9006: connection terminal, 9007: sensor, 9008: microphone, 9050:operation button, 9051: information, 9052: information, 9053:information, 9054: information, 9055: hinge, 9100: portable informationterminal, 9101: portable information terminal, 9102: portableinformation terminal, 9200: portable information terminal, 9201:portable information terminal, 9300: television device, 9301: stand,9311: remote controller, 9500: display device, 9501: display panel,9502: display region, 9503: region, 9511: axis portion, 9512: bearing,9700: automobile, 9701: car body, 9702: wheel, 9703: dashboard, 9704:light, 9710: display portion, 9711: display portion, 9712: displayportion, 9713: display portion, 9714: display portion, 9715: displayportion, 9721: display portion, 9722: display portion, 9723: displayportion.

This application is based on Japanese Patent Application serial no.2015-194744 filed with Japan Patent Office on Sep. 30, 2015 and JapanesePatent Application serial no. 2015-237266 filed with Japan Patent Officeon Dec. 4, 2015, the entire contents of which are hereby incorporated byreference.

1. A light-emitting element comprising: a pair of electrodes; and alayer between the pair of electrodes, the layer comprising a guestmaterial and a host material, wherein the guest material is capable ofconverting triplet excitation energy into light emission, wherein a HOMOlevel of the guest material is higher than a HOMO level of the hostmaterial, wherein the guest material has an emission peak in a redwavelength range in an emission spectrum, and wherein an energydifference between a LUMO level of the host material and the HOMO levelof the guest material is larger than or equal to transition energycalculated from an absorption edge of an absorption spectrum of theguest material.
 2. The light-emitting element according to claim 1,wherein an energy difference between a LUMO level of the guest materialand the HOMO level of the guest material is larger than the transitionenergy calculated from the absorption edge of the absorption spectrum ofthe guest material by 0.3 eV to 0.8 eV.
 3. The light-emitting elementaccording to claim 1, wherein the host material has a difference betweena singlet excitation energy level and a triplet excitation energy levelof larger than 0 eV and smaller than or equal to 0.2 eV.
 4. Thelight-emitting element according to claim 1, wherein the host materialis capable of exhibiting thermally activated delayed fluorescence atroom temperature.
 5. The light-emitting element according to claim 1,wherein the host material is capable of supplying excitation energy tothe guest material.
 6. The light-emitting element according to claim 1,wherein an emission spectrum of the host material comprises a wavelengthregion overlapping with an absorption band on lowest energy side in theabsorption spectrum of the guest material.
 7. The light-emitting elementaccording to claim 1, wherein the guest material comprises iridium. 8.The light-emitting element according to claim 1, wherein the guestmaterial is an organometallic complex having an isoquinoline ligand. 9.The light-emitting element according to claim 1, wherein the hostmaterial is capable of transporting an electron and a hole.
 10. Thelight-emitting element according to claim 1, wherein the host materialcomprises a diazine skeleton such as a pyrimidine skeleton, a pyrazineskeleton, or a pyridazine skeleton and a pyrrole skeleton such as anindole skeleton, a carbazole skeleton, or a9-phenyl-3,3′-bi-9H-carbazole skeleton.
 11. The light-emitting elementaccording to claim 10, wherein the diazine skeleton is directly bondedto the pyrrole skeleton.
 12. The light-emitting element according toclaim 1, wherein the red wavelength range is greater than or equal to580 nm and less than or equal to 680 nm, and wherein the guest materialhas at least one peak in the emission spectrum.
 13. The light-emittingelement according to claim 1, wherein an energy difference between theHOMO level of the guest material and the HOMO level of the host materialis greater than or equal to 0.05 eV and less than or equal to 0.4 eV.14. The light-emitting element according to claim 1, wherein an energydifference between the LUMO level of the guest material and the LUMOlevel of the host material is greater than or equal to 0.2 eV.
 15. Alight-emitting element comprising: a pair of electrodes; and a layerbetween the pair of electrodes, the layer comprising a guest materialand a host material, wherein the guest material is capable of convertingtriplet excitation energy into light emission, wherein a HOMO level ofthe guest material is higher than a HOMO level of the host material,wherein the guest material has an emission peak in a red wavelengthrange in an emission spectrum, and wherein an energy difference betweena LUMO level of the host material and the HOMO level of the guestmaterial is larger than or equal to light emission energy of the guestmaterial.
 16. The light-emitting element according to claim 15, whereinan energy difference between a LUMO level of the guest material and theHOMO level of the guest material is larger than a transition energycalculated from an absorption edge of an absorption spectrum of theguest material by 0.3 eV to 0.8 eV.
 17. The light-emitting elementaccording to claim 15, wherein an energy difference between a LUMO levelof the guest material and the HOMO level of the guest material is largerthan the light emission energy of the guest material by 0.3 eV to 0.8eV.
 18. The light-emitting element according to claim 15, wherein thehost material has a difference between a singlet excitation energy leveland a triplet excitation energy level of larger than 0 eV and smallerthan or equal to 0.2 eV.
 19. The light-emitting element according toclaim 15, wherein the host material is capable of exhibiting thermallyactivated delayed fluorescence at room temperature.
 20. Thelight-emitting element according to claim 15, wherein the host materialis capable of supplying excitation energy to the guest material.
 21. Thelight-emitting element according to claim 15, wherein an emissionspectrum of the host material comprises a wavelength region overlappingwith an absorption band on lowest energy side in the absorption spectrumof the guest material.
 22. The light-emitting element according to claim15, wherein the guest material comprises iridium.
 23. The light-emittingelement according to claim 15, wherein the guest material is anorganometallic complex having an isoquinoline ligand.
 24. Thelight-emitting element according to claim 15, wherein the host materialis capable of transporting an electron and a hole.
 25. Thelight-emitting element according to claim 15, wherein the host materialcomprises a diazine skeleton such as a pyrimidine skeleton, a pyrazineskeleton, or a pyridazine skeleton and a pyrrole skeleton such as anindole skeleton, a carbazole skeleton, or a9-phenyl-3,3′-bi-9H-carbazole skeleton.
 26. The light-emitting elementaccording to claim 25, wherein the diazine skeleton is directly bondedto the pyrrole skeleton.
 27. The light-emitting element according toclaim 15, wherein the red wavelength range is greater than or equal to580 nm and less than or equal to 680 nm, and wherein the guest materialhas at least one peak in the emission spectrum.
 28. The light-emittingelement according to claim 15, wherein an energy difference between theHOMO level of the guest material and the HOMO level of the host materialis greater than or equal to 0.05 eV and less than or equal to 0.4 eV.29. The light-emitting element according to claim 15, wherein an energydifference between the LUMO level of the guest material and the LUMOlevel of the host material is greater than or equal to 0.2 eV.